implements PR007.1 clean up

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsCallableScope.java

@@ -0,0 +1,33 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.source.identifiers.NameId;
+
+record PbsCallableScope(
+        String label,
+        String scopeKind,
+        NameId ownerNameId) {
+
+    static PbsCallableScope topLevelFunctions() {
+        return new PbsCallableScope("top-level callable scope", "top-level", null);
+    }
+
+    static PbsCallableScope structMethods(final NameId ownerNameId) {
+        return new PbsCallableScope("struct method scope", "struct", ownerNameId);
+    }
+
+    static PbsCallableScope structCtors(final NameId ownerNameId) {
+        return new PbsCallableScope("struct constructor scope", "ctor", ownerNameId);
+    }
+
+    static PbsCallableScope serviceMethods(final NameId ownerNameId) {
+        return new PbsCallableScope("service method scope", "service", ownerNameId);
+    }
+
+    static PbsCallableScope contractSignatures(final NameId ownerNameId) {
+        return new PbsCallableScope("contract signature scope", "contract", ownerNameId);
+    }
+
+    static PbsCallableScope implementsMethods(final NameId ownerNameId) {
+        return new PbsCallableScope("implements method scope", "implements", ownerNameId);
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsCallableShape.java

@@ -0,0 +1,91 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.pbs.ast.PbsAst;
+import p.studio.utilities.structures.ReadOnlyList;
+
+final class PbsCallableShape {
+    private PbsCallableShape() {
+    }
+
+    static String callableShapeKey(
+            final ReadOnlyList<PbsAst.Parameter> parameters,
+            final PbsAst.ReturnKind returnKind,
+            final PbsAst.TypeRef returnType,
+            final PbsAst.TypeRef resultErrorType) {
+        final var builder = new StringBuilder();
+        builder.append('(');
+        for (int i = 0; i < parameters.size(); i++) {
+            if (i > 0) {
+                builder.append(',');
+            }
+            builder.append(typeKey(parameters.get(i).typeRef()));
+        }
+        builder.append(")->");
+        builder.append(outputSurfaceKey(returnKind, returnType, resultErrorType));
+        return builder.toString();
+    }
+
+    static String ctorShapeKey(final ReadOnlyList<PbsAst.Parameter> parameters) {
+        final var builder = new StringBuilder();
+        builder.append('(');
+        for (int i = 0; i < parameters.size(); i++) {
+            if (i > 0) {
+                builder.append(',');
+            }
+            builder.append(typeKey(parameters.get(i).typeRef()));
+        }
+        builder.append(')');
+        return builder.toString();
+    }
+
+    private static String outputSurfaceKey(
+            final PbsAst.ReturnKind returnKind,
+            final PbsAst.TypeRef returnType,
+            final PbsAst.TypeRef resultErrorType) {
+        return switch (returnKind) {
+            case INFERRED_UNIT, EXPLICIT_UNIT -> "unit";
+            case PLAIN -> carrierTypeKey(returnType);
+            case RESULT -> "result<" + typeKey(resultErrorType) + ">" + carrierTypeKey(returnType);
+        };
+    }
+
+    private static String carrierTypeKey(final PbsAst.TypeRef typeRef) {
+        final var unwrapped = unwrapGroup(typeRef);
+        if (unwrapped != null && unwrapped.kind() == PbsAst.TypeRefKind.NAMED_TUPLE && unwrapped.fields().size() == 1) {
+            return typeKey(unwrapped.fields().getFirst().typeRef());
+        }
+        return typeKey(unwrapped);
+    }
+
+    private static PbsAst.TypeRef unwrapGroup(final PbsAst.TypeRef typeRef) {
+        if (typeRef == null) {
+            return null;
+        }
+        if (typeRef.kind() != PbsAst.TypeRefKind.GROUP) {
+            return typeRef;
+        }
+        return unwrapGroup(typeRef.inner());
+    }
+
+    private static String typeKey(final PbsAst.TypeRef typeRef) {
+        if (typeRef == null) {
+            return "unit";
+        }
+        final var unwrapped = unwrapGroup(typeRef);
+        if (unwrapped == null) {
+            return "unit";
+        }
+        return switch (unwrapped.kind()) {
+            case SIMPLE -> "simple:" + unwrapped.name();
+            case SELF -> "self";
+            case OPTIONAL -> "optional(" + typeKey(unwrapped.inner()) + ")";
+            case UNIT -> "unit";
+            case GROUP -> typeKey(unwrapped.inner());
+            case NAMED_TUPLE -> "tuple(" + unwrapped.fields().stream()
+                    .map(field -> typeKey(field.typeRef()))
+                    .reduce((left, right) -> left + "," + right)
+                    .orElse("") + ")";
+            case ERROR -> "error";
+        };
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsCtorReturnScanner.java

@@ -0,0 +1,74 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.pbs.ast.PbsAst;
+
+final class PbsCtorReturnScanner {
+    private PbsCtorReturnScanner() {
+    }
+
+    static boolean containsReturnStatement(final PbsAst.Block block) {
+        if (block == null) {
+            return false;
+        }
+
+        for (final var statement : block.statements()) {
+            if (statement instanceof PbsAst.ReturnStatement) {
+                return true;
+            }
+            if (statement instanceof PbsAst.IfStatement ifStatement) {
+                if (containsReturnInIfStatement(ifStatement)) {
+                    return true;
+                }
+                continue;
+            }
+            if (statement instanceof PbsAst.ForStatement forStatement && containsReturnStatement(forStatement.body())) {
+                return true;
+            }
+            if (statement instanceof PbsAst.WhileStatement whileStatement && containsReturnStatement(whileStatement.body())) {
+                return true;
+            }
+            if (statement instanceof PbsAst.ExpressionStatement expressionStatement && containsReturnExpression(expressionStatement.expression())) {
+                return true;
+            }
+        }
+
+        return block.tailExpression() != null && containsReturnExpression(block.tailExpression());
+    }
+
+    private static boolean containsReturnInIfStatement(final PbsAst.IfStatement ifStatement) {
+        if (ifStatement == null) {
+            return false;
+        }
+        if (containsReturnStatement(ifStatement.thenBlock()) || containsReturnStatement(ifStatement.elseBlock())) {
+            return true;
+        }
+        return containsReturnInIfStatement(ifStatement.elseIf());
+    }
+
+    private static boolean containsReturnExpression(final PbsAst.Expression expression) {
+        if (expression == null) {
+            return false;
+        }
+        if (expression instanceof PbsAst.BlockExpr blockExpr) {
+            return containsReturnStatement(blockExpr.block());
+        }
+        if (expression instanceof PbsAst.IfExpr ifExpr) {
+            return containsReturnStatement(ifExpr.thenBlock()) || containsReturnExpression(ifExpr.elseExpression());
+        }
+        if (expression instanceof PbsAst.SwitchExpr switchExpr) {
+            for (final var arm : switchExpr.arms()) {
+                if (containsReturnStatement(arm.block())) {
+                    return true;
+                }
+            }
+        }
+        if (expression instanceof PbsAst.HandleExpr handleExpr) {
+            for (final var arm : handleExpr.arms()) {
+                if (containsReturnStatement(arm.block())) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsDeclarationRuleValidator.java

@@ -0,0 +1,187 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.pbs.ast.PbsAst;
+import p.studio.compiler.source.Span;
+import p.studio.compiler.source.diagnostics.DiagnosticSink;
+import p.studio.compiler.source.diagnostics.RelatedSpan;
+import p.studio.compiler.source.diagnostics.Severity;
+import p.studio.compiler.source.identifiers.NameId;
+import p.studio.compiler.source.tables.NameTable;
+import p.studio.utilities.structures.ReadOnlyList;
+
+import java.util.HashMap;
+import java.util.List;
+
+final class PbsDeclarationRuleValidator {
+    private final NameTable nameTable;
+    private final DiagnosticSink diagnostics;
+
+    PbsDeclarationRuleValidator(
+            final NameTable nameTable,
+            final DiagnosticSink diagnostics) {
+        this.nameTable = nameTable;
+        this.diagnostics = diagnostics;
+    }
+
+    void validateCallableHeader(
+            final String callableName,
+            final ReadOnlyList<PbsAst.Parameter> parameters,
+            final PbsAst.ReturnKind returnKind,
+            final PbsAst.TypeRef returnType,
+            final PbsAst.TypeRef resultErrorType,
+            final Span span) {
+        validateParameters(parameters, "callable '%s'".formatted(callableName));
+        validateReturnSurface(returnKind, returnType, resultErrorType, "callable '%s'".formatted(callableName), span);
+        PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(
+                parameters.stream().map(PbsAst.Parameter::typeRef).toList(),
+                "callable '%s'".formatted(callableName),
+                diagnostics);
+    }
+
+    void validateParameters(
+            final ReadOnlyList<PbsAst.Parameter> parameters,
+            final String ownerDescription) {
+        final var seen = new HashMap<NameId, Span>();
+        for (final var parameter : parameters) {
+            final var parameterNameId = nameTable.register(parameter.name());
+            final var first = seen.putIfAbsent(parameterNameId, parameter.span());
+            if (first != null) {
+                diagnostics.report(
+                        Severity.Error,
+                        PbsSemanticsErrors.E_SEM_DUPLICATE_PARAMETER_NAME.name(),
+                        "Duplicate parameter name '%s' in %s".formatted(parameter.name(), ownerDescription),
+                        parameter.span(),
+                        List.of(new RelatedSpan("First parameter is here", first)));
+            }
+        }
+    }
+
+    void validateTypeSurfaces(
+            final List<PbsAst.TypeRef> typeRoots,
+            final String ownerDescription) {
+        PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(typeRoots, ownerDescription, diagnostics);
+    }
+
+    void validateErrorDeclaration(final PbsAst.ErrorDecl errorDecl) {
+        final var seenCases = new HashMap<NameId, Span>();
+        for (final var caseName : errorDecl.cases()) {
+            final var caseNameId = nameTable.register(caseName);
+            final var first = seenCases.putIfAbsent(caseNameId, errorDecl.span());
+            if (first != null) {
+                diagnostics.report(
+                        Severity.Error,
+                        PbsSemanticsErrors.E_SEM_DUPLICATE_ERROR_CASE_LABEL.name(),
+                        "Duplicate error case label '%s' in '%s'".formatted(caseName, errorDecl.name()),
+                        errorDecl.span(),
+                        List.of(new RelatedSpan("First case is here", first)));
+            }
+        }
+    }
+
+    void validateEnumDeclaration(final PbsAst.EnumDecl enumDecl) {
+        final var seenLabels = new HashMap<NameId, Span>();
+        final var seenIds = new HashMap<Long, Span>();
+        for (final var enumCase : enumDecl.cases()) {
+            final var labelId = nameTable.register(enumCase.name());
+            final var firstLabel = seenLabels.putIfAbsent(labelId, enumCase.span());
+            if (firstLabel != null) {
+                diagnostics.report(
+                        Severity.Error,
+                        PbsSemanticsErrors.E_SEM_DUPLICATE_ENUM_CASE_LABEL.name(),
+                        "Duplicate enum case label '%s' in '%s'".formatted(enumCase.name(), enumDecl.name()),
+                        enumCase.span(),
+                        List.of(new RelatedSpan("First case is here", firstLabel)));
+            }
+
+            if (enumCase.explicitValue() != null) {
+                final var firstId = seenIds.putIfAbsent(enumCase.explicitValue(), enumCase.span());
+                if (firstId != null) {
+                    diagnostics.report(
+                            Severity.Error,
+                            PbsSemanticsErrors.E_SEM_DUPLICATE_ENUM_CASE_ID.name(),
+                            "Duplicate enum case id '%s' in '%s'".formatted(enumCase.explicitValue(), enumDecl.name()),
+                            enumCase.span(),
+                            List.of(new RelatedSpan("First id is here", firstId)));
+                }
+            }
+        }
+    }
+
+    void validateCallbackDeclaration(final PbsAst.CallbackDecl callbackDecl) {
+        validateParameters(callbackDecl.parameters(), "callback '%s'".formatted(callbackDecl.name()));
+        validateReturnSurface(
+                callbackDecl.returnKind(),
+                callbackDecl.returnType(),
+                callbackDecl.resultErrorType(),
+                "callback '%s'".formatted(callbackDecl.name()),
+                callbackDecl.span());
+        PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(
+                callbackDecl.parameters().stream().map(PbsAst.Parameter::typeRef).toList(),
+                "callback '%s'".formatted(callbackDecl.name()),
+                diagnostics);
+    }
+
+    void validateConstDeclaration(final PbsAst.ConstDecl constDecl) {
+        if (constDecl.explicitType() == null) {
+            diagnostics.error(
+                    PbsSemanticsErrors.E_SEM_MISSING_CONST_TYPE_ANNOTATION.name(),
+                    "Const declaration '%s' must include an explicit type annotation".formatted(constDecl.name()),
+                    constDecl.span());
+        } else {
+            PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(
+                    List.of(constDecl.explicitType()),
+                    "const '%s'".formatted(constDecl.name()),
+                    diagnostics);
+        }
+
+        if (constDecl.initializer() == null) {
+            diagnostics.error(
+                    PbsSemanticsErrors.E_SEM_MISSING_CONST_INITIALIZER.name(),
+                    "Non-builtin const declaration '%s' must include an initializer".formatted(constDecl.name()),
+                    constDecl.span());
+        }
+    }
+
+    private void validateReturnSurface(
+            final PbsAst.ReturnKind returnKind,
+            final PbsAst.TypeRef returnType,
+            final PbsAst.TypeRef resultErrorType,
+            final String ownerDescription,
+            final Span span) {
+        if (returnKind == PbsAst.ReturnKind.RESULT && PbsTypeSurfaceSemanticsValidator.isOptionalType(returnType)) {
+            diagnostics.error(
+                    PbsSemanticsErrors.E_SEM_INVALID_MIXED_OPTIONAL_RESULT_RETURN.name(),
+                    "Return surface for %s cannot combine 'result' with top-level 'optional' payload".formatted(ownerDescription),
+                    span);
+        }
+
+        if (returnType != null && returnType.kind() == PbsAst.TypeRefKind.NAMED_TUPLE) {
+            validateDuplicateOutputLabels(returnType, ownerDescription);
+        }
+
+        if (returnType != null) {
+            PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(List.of(returnType), ownerDescription, diagnostics);
+        }
+        if (resultErrorType != null) {
+            PbsTypeSurfaceSemanticsValidator.validateTypeSurfaceRecursive(List.of(resultErrorType), ownerDescription, diagnostics);
+        }
+    }
+
+    private void validateDuplicateOutputLabels(
+            final PbsAst.TypeRef namedTupleReturnType,
+            final String ownerDescription) {
+        final var seen = new HashMap<NameId, Span>();
+        for (final var field : namedTupleReturnType.fields()) {
+            final var labelId = nameTable.register(field.label());
+            final var first = seen.putIfAbsent(labelId, field.span());
+            if (first != null) {
+                diagnostics.report(
+                        Severity.Error,
+                        PbsSemanticsErrors.E_SEM_DUPLICATE_RETURN_LABEL.name(),
+                        "Duplicate return label '%s' in %s".formatted(field.label(), ownerDescription),
+                        field.span(),
+                        List.of(new RelatedSpan("First label is here", first)));
+            }
+        }
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsDeclarationSemanticsValidator.java

@@ -3,27 +3,20 @@ package p.studio.compiler.pbs.semantics;
 import p.studio.compiler.pbs.ast.PbsAst;
 import p.studio.compiler.source.Span;
 import p.studio.compiler.source.diagnostics.DiagnosticSink;
-import p.studio.compiler.source.diagnostics.RelatedSpan;
-import p.studio.compiler.source.diagnostics.Severity;
-import p.studio.compiler.source.identifiers.NameId;
 import p.studio.compiler.source.tables.NameTable;
 import p.studio.utilities.structures.ReadOnlyList;
 
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-
 public final class PbsDeclarationSemanticsValidator {
     private final NameTable nameTable = new NameTable();
 
     public void validate(final PbsAst.File ast, final DiagnosticSink diagnostics) {
-        final var binder = new NamespaceBinder(nameTable, diagnostics);
+        final var binder = new PbsNamespaceBinder(nameTable, diagnostics);
+        final var rules = new PbsDeclarationRuleValidator(nameTable, diagnostics);
 
         for (final var topDecl : ast.topDecls()) {
             if (topDecl instanceof PbsAst.FunctionDecl functionDecl) {
                 validateCallable(
-                        CallableScope.topLevelFunctions(),
+                        PbsCallableScope.topLevelFunctions(),
                         functionDecl.name(),
                         functionDecl.parameters(),
                         functionDecl.returnKind(),
@@ -31,64 +24,66 @@ public final class PbsDeclarationSemanticsValidator {
                         functionDecl.resultErrorType(),
                         functionDecl.span(),
                         binder,
-                        diagnostics);
+                        rules);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.StructDecl structDecl) {
                 binder.registerType(structDecl.name(), structDecl.span(), "struct");
-                validateStructDeclaration(structDecl, binder, diagnostics);
+                validateStructDeclaration(structDecl, binder, rules, diagnostics);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.ServiceDecl serviceDecl) {
                 binder.registerType(serviceDecl.name(), serviceDecl.span(), "service");
                 binder.registerValue(serviceDecl.name(), serviceDecl.span(), "service singleton");
-                validateServiceDeclaration(serviceDecl, binder, diagnostics);
+                validateServiceDeclaration(serviceDecl, binder, rules);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.ContractDecl contractDecl) {
                 binder.registerType(contractDecl.name(), contractDecl.span(), "contract");
-                validateContractDeclaration(contractDecl, binder, diagnostics);
+                validateContractDeclaration(contractDecl, binder, rules);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.ErrorDecl errorDecl) {
                 binder.registerType(errorDecl.name(), errorDecl.span(), "error");
-                validateErrorDeclaration(errorDecl, diagnostics);
+                rules.validateErrorDeclaration(errorDecl);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.EnumDecl enumDecl) {
                 binder.registerType(enumDecl.name(), enumDecl.span(), "enum");
-                validateEnumDeclaration(enumDecl, diagnostics);
+                rules.validateEnumDeclaration(enumDecl);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.CallbackDecl callbackDecl) {
                 binder.registerType(callbackDecl.name(), callbackDecl.span(), "callback");
-                validateCallbackDeclaration(callbackDecl, diagnostics);
+                rules.validateCallbackDeclaration(callbackDecl);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.ConstDecl constDecl) {
                 binder.registerValue(constDecl.name(), constDecl.span(), "const");
-                validateConstDeclaration(constDecl, diagnostics);
+                rules.validateConstDeclaration(constDecl);
                 continue;
             }
 
             if (topDecl instanceof PbsAst.ImplementsDecl implementsDecl) {
-                validateImplementsDeclaration(implementsDecl, binder, diagnostics);
+                validateImplementsDeclaration(implementsDecl, binder, rules);
             }
         }
     }
 
     private void validateStructDeclaration(
             final PbsAst.StructDecl structDecl,
-            final NamespaceBinder binder,
+            final PbsNamespaceBinder binder,
+            final PbsDeclarationRuleValidator rules,
             final DiagnosticSink diagnostics) {
-        final var methodScope = CallableScope.structMethods(nameTable.register(structDecl.name()));
+        final var ownerNameId = nameTable.register(structDecl.name());
+        final var methodScope = PbsCallableScope.structMethods(ownerNameId);
         for (final var method : structDecl.methods()) {
             validateCallable(
                     methodScope,
@@ -99,19 +94,18 @@ public final class PbsDeclarationSemanticsValidator {
                     method.resultErrorType(),
                     method.span(),
                     binder,
-                    diagnostics);
+                    rules);
         }
 
-        final var ctorScope = CallableScope.structCtors(nameTable.register(structDecl.name()));
+        final var ctorScope = PbsCallableScope.structCtors(ownerNameId);
         for (final var ctor : structDecl.ctors()) {
-            validateParameters(ctor.parameters(), "ctor '%s'".formatted(ctor.name()), diagnostics);
+            rules.validateParameters(ctor.parameters(), "ctor '%s'".formatted(ctor.name()));
-            validateTypeSurfaceRecursive(
+            rules.validateTypeSurfaces(
                     ctor.parameters().stream().map(PbsAst.Parameter::typeRef).toList(),
-                    "ctor '%s'".formatted(ctor.name()),
+                    "ctor '%s'".formatted(ctor.name()));
-                    diagnostics);
 
-            binder.registerCtor(ctorScope, ctor.name(), ctorShapeKey(ctor.parameters()), ctor.span());
+            binder.registerCtor(ctorScope, ctor.name(), PbsCallableShape.ctorShapeKey(ctor.parameters()), ctor.span());
-            if (containsReturnStatement(ctor.body())) {
+            if (PbsCtorReturnScanner.containsReturnStatement(ctor.body())) {
                 diagnostics.error(
                         PbsSemanticsErrors.E_SEM_INVALID_RETURN_INSIDE_CTOR.name(),
                         "Constructors cannot contain 'return' statements",
@@ -122,9 +116,9 @@ public final class PbsDeclarationSemanticsValidator {
 
     private void validateServiceDeclaration(
             final PbsAst.ServiceDecl serviceDecl,
-            final NamespaceBinder binder,
+            final PbsNamespaceBinder binder,
-            final DiagnosticSink diagnostics) {
+            final PbsDeclarationRuleValidator rules) {
-        final var methodScope = CallableScope.serviceMethods(nameTable.register(serviceDecl.name()));
+        final var methodScope = PbsCallableScope.serviceMethods(nameTable.register(serviceDecl.name()));
         for (final var method : serviceDecl.methods()) {
             validateCallable(
                     methodScope,
@@ -135,15 +129,15 @@ public final class PbsDeclarationSemanticsValidator {
                     method.resultErrorType(),
                     method.span(),
                     binder,
-                    diagnostics);
+                    rules);
         }
     }
 
     private void validateContractDeclaration(
             final PbsAst.ContractDecl contractDecl,
-            final NamespaceBinder binder,
+            final PbsNamespaceBinder binder,
-            final DiagnosticSink diagnostics) {
+            final PbsDeclarationRuleValidator rules) {
-        final var signatureScope = CallableScope.contractSignatures(nameTable.register(contractDecl.name()));
+        final var signatureScope = PbsCallableScope.contractSignatures(nameTable.register(contractDecl.name()));
         for (final var signature : contractDecl.signatures()) {
             validateCallable(
                     signatureScope,
@@ -154,105 +148,16 @@ public final class PbsDeclarationSemanticsValidator {
                     signature.resultErrorType(),
                     signature.span(),
                     binder,
-                    diagnostics);
+                    rules);
-        }
-    }
-
-    private void validateErrorDeclaration(
-            final PbsAst.ErrorDecl errorDecl,
-            final DiagnosticSink diagnostics) {
-        final var seenCases = new HashMap<NameId, Span>();
-        for (final var caseName : errorDecl.cases()) {
-            final var caseNameId = nameTable.register(caseName);
-            final var first = seenCases.putIfAbsent(caseNameId, errorDecl.span());
-            if (first != null) {
-                diagnostics.report(
-                        Severity.Error,
-                        PbsSemanticsErrors.E_SEM_DUPLICATE_ERROR_CASE_LABEL.name(),
-                        "Duplicate error case label '%s' in '%s'".formatted(caseName, errorDecl.name()),
-                        errorDecl.span(),
-                        List.of(new RelatedSpan("First case is here", first)));
-            }
-        }
-    }
-
-    private void validateEnumDeclaration(
-            final PbsAst.EnumDecl enumDecl,
-            final DiagnosticSink diagnostics) {
-        final var seenLabels = new HashMap<NameId, Span>();
-        final var seenIds = new HashMap<Long, Span>();
-        for (final var enumCase : enumDecl.cases()) {
-            final var labelId = nameTable.register(enumCase.name());
-            final var firstLabel = seenLabels.putIfAbsent(labelId, enumCase.span());
-            if (firstLabel != null) {
-                diagnostics.report(
-                        Severity.Error,
-                        PbsSemanticsErrors.E_SEM_DUPLICATE_ENUM_CASE_LABEL.name(),
-                        "Duplicate enum case label '%s' in '%s'".formatted(enumCase.name(), enumDecl.name()),
-                        enumCase.span(),
-                        List.of(new RelatedSpan("First case is here", firstLabel)));
-            }
-
-            if (enumCase.explicitValue() != null) {
-                final var firstId = seenIds.putIfAbsent(enumCase.explicitValue(), enumCase.span());
-                if (firstId != null) {
-                    diagnostics.report(
-                            Severity.Error,
-                            PbsSemanticsErrors.E_SEM_DUPLICATE_ENUM_CASE_ID.name(),
-                            "Duplicate enum case id '%s' in '%s'".formatted(enumCase.explicitValue(), enumDecl.name()),
-                            enumCase.span(),
-                            List.of(new RelatedSpan("First id is here", firstId)));
-                }
-            }
-        }
-    }
-
-    private void validateCallbackDeclaration(
-            final PbsAst.CallbackDecl callbackDecl,
-            final DiagnosticSink diagnostics) {
-        validateParameters(callbackDecl.parameters(), "callback '%s'".formatted(callbackDecl.name()), diagnostics);
-        validateReturnSurface(
-                callbackDecl.returnKind(),
-                callbackDecl.returnType(),
-                callbackDecl.resultErrorType(),
-                "callback '%s'".formatted(callbackDecl.name()),
-                callbackDecl.span(),
-                diagnostics);
-        validateTypeSurfaceRecursive(
-                callbackDecl.parameters().stream().map(PbsAst.Parameter::typeRef).toList(),
-                "callback '%s'".formatted(callbackDecl.name()),
-                diagnostics);
-    }
-
-    private void validateConstDeclaration(
-            final PbsAst.ConstDecl constDecl,
-            final DiagnosticSink diagnostics) {
-        if (constDecl.explicitType() == null) {
-            diagnostics.error(
-                    PbsSemanticsErrors.E_SEM_MISSING_CONST_TYPE_ANNOTATION.name(),
-                    "Const declaration '%s' must include an explicit type annotation".formatted(constDecl.name()),
-                    constDecl.span());
-        } else {
-            validateTypeSurfaceRecursive(
-                    List.of(constDecl.explicitType()),
-                    "const '%s'".formatted(constDecl.name()),
-                    diagnostics);
-        }
-
-        if (constDecl.initializer() == null) {
-            diagnostics.error(
-                    PbsSemanticsErrors.E_SEM_MISSING_CONST_INITIALIZER.name(),
-                    "Non-builtin const declaration '%s' must include an initializer".formatted(constDecl.name()),
-                    constDecl.span());
         }
     }
 
     private void validateImplementsDeclaration(
             final PbsAst.ImplementsDecl implementsDecl,
-            final NamespaceBinder binder,
+            final PbsNamespaceBinder binder,
-            final DiagnosticSink diagnostics) {
+            final PbsDeclarationRuleValidator rules) {
         final var ownerNameId = nameTable.register(implementsDecl.ownerName() + ":" + implementsDecl.contractName());
-        final var scope = CallableScope.implementsMethods(ownerNameId);
+        final var scope = PbsCallableScope.implementsMethods(ownerNameId);
         for (final var method : implementsDecl.methods()) {
             validateCallable(
                     scope,
@@ -263,407 +168,25 @@ public final class PbsDeclarationSemanticsValidator {
                     method.resultErrorType(),
                     method.span(),
                     binder,
-                    diagnostics);
+                    rules);
         }
     }
 
     private void validateCallable(
-            final CallableScope scope,
+            final PbsCallableScope scope,
             final String callableName,
             final ReadOnlyList<PbsAst.Parameter> parameters,
             final PbsAst.ReturnKind returnKind,
             final PbsAst.TypeRef returnType,
             final PbsAst.TypeRef resultErrorType,
             final Span span,
-            final NamespaceBinder binder,
+            final PbsNamespaceBinder binder,
-            final DiagnosticSink diagnostics) {
+            final PbsDeclarationRuleValidator rules) {
-        validateParameters(parameters, "callable '%s'".formatted(callableName), diagnostics);
+        rules.validateCallableHeader(callableName, parameters, returnKind, returnType, resultErrorType, span);
-        validateReturnSurface(returnKind, returnType, resultErrorType, "callable '%s'".formatted(callableName), span, diagnostics);
+        binder.registerCallable(
-        validateTypeSurfaceRecursive(parameters.stream().map(PbsAst.Parameter::typeRef).toList(), "callable '%s'".formatted(callableName), diagnostics);
+                scope,
-
+                callableName,
-        binder.registerCallable(scope, callableName, callableShapeKey(parameters, returnKind, returnType, resultErrorType), span);
+                PbsCallableShape.callableShapeKey(parameters, returnKind, returnType, resultErrorType),
-    }
-
-    private void validateParameters(
-            final ReadOnlyList<PbsAst.Parameter> parameters,
-            final String ownerDescription,
-            final DiagnosticSink diagnostics) {
-        final var seen = new HashMap<NameId, Span>();
-        for (final var parameter : parameters) {
-            final var parameterNameId = nameTable.register(parameter.name());
-            final var first = seen.putIfAbsent(parameterNameId, parameter.span());
-            if (first != null) {
-                diagnostics.report(
-                        Severity.Error,
-                        PbsSemanticsErrors.E_SEM_DUPLICATE_PARAMETER_NAME.name(),
-                        "Duplicate parameter name '%s' in %s".formatted(parameter.name(), ownerDescription),
-                        parameter.span(),
-                        List.of(new RelatedSpan("First parameter is here", first)));
-            }
-        }
-    }
-
-    private void validateReturnSurface(
-            final PbsAst.ReturnKind returnKind,
-            final PbsAst.TypeRef returnType,
-            final PbsAst.TypeRef resultErrorType,
-            final String ownerDescription,
-            final Span span,
-            final DiagnosticSink diagnostics) {
-        if (returnKind == PbsAst.ReturnKind.RESULT && isOptionalType(returnType)) {
-            diagnostics.error(
-                    PbsSemanticsErrors.E_SEM_INVALID_MIXED_OPTIONAL_RESULT_RETURN.name(),
-                    "Return surface for %s cannot combine 'result' with top-level 'optional' payload".formatted(ownerDescription),
                 span);
     }
-
-        if (returnType != null && returnType.kind() == PbsAst.TypeRefKind.NAMED_TUPLE) {
-            validateDuplicateOutputLabels(returnType, ownerDescription, diagnostics);
-        }
-
-        if (returnType != null) {
-            validateTypeSurfaceRecursive(List.of(returnType), ownerDescription, diagnostics);
-        }
-        if (resultErrorType != null) {
-            validateTypeSurfaceRecursive(List.of(resultErrorType), ownerDescription, diagnostics);
-        }
-    }
-
-    private void validateDuplicateOutputLabels(
-            final PbsAst.TypeRef namedTupleReturnType,
-            final String ownerDescription,
-            final DiagnosticSink diagnostics) {
-        final var seen = new HashMap<NameId, Span>();
-        for (final var field : namedTupleReturnType.fields()) {
-            final var labelId = nameTable.register(field.label());
-            final var first = seen.putIfAbsent(labelId, field.span());
-            if (first != null) {
-                diagnostics.report(
-                        Severity.Error,
-                        PbsSemanticsErrors.E_SEM_DUPLICATE_RETURN_LABEL.name(),
-                        "Duplicate return label '%s' in %s".formatted(field.label(), ownerDescription),
-                        field.span(),
-                        List.of(new RelatedSpan("First label is here", first)));
-            }
-        }
-    }
-
-    private void validateTypeSurfaceRecursive(
-            final List<PbsAst.TypeRef> roots,
-            final String ownerDescription,
-            final DiagnosticSink diagnostics) {
-        final var pending = new ArrayList<>(roots);
-        while (!pending.isEmpty()) {
-            final var typeRef = pending.removeLast();
-            if (typeRef == null) {
-                continue;
-            }
-
-            switch (typeRef.kind()) {
-                case OPTIONAL -> {
-                    if (typeRef.inner() == null || typeRef.inner().kind() == PbsAst.TypeRefKind.UNIT) {
-                        diagnostics.error(
-                                PbsSemanticsErrors.E_SEM_INVALID_OPTIONAL_VOID_TYPE_SURFACE.name(),
-                                "Invalid optional type surface in %s: 'optional void' is not allowed".formatted(ownerDescription),
-                                typeRef.span());
-                    }
-                    pending.add(typeRef.inner());
-                }
-                case GROUP -> pending.add(typeRef.inner());
-                case NAMED_TUPLE -> {
-                    for (final var field : typeRef.fields()) {
-                        pending.add(field.typeRef());
-                    }
-                }
-                default -> {
-                    // No recursive children.
-                }
-            }
-        }
-    }
-
-    private boolean isOptionalType(final PbsAst.TypeRef typeRef) {
-        if (typeRef == null) {
-            return false;
-        }
-        if (typeRef.kind() == PbsAst.TypeRefKind.OPTIONAL) {
-            return true;
-        }
-        if (typeRef.kind() == PbsAst.TypeRefKind.GROUP) {
-            return isOptionalType(typeRef.inner());
-        }
-        return false;
-    }
-
-    private boolean containsReturnStatement(final PbsAst.Block block) {
-        if (block == null) {
-            return false;
-        }
-
-        for (final var statement : block.statements()) {
-            if (statement instanceof PbsAst.ReturnStatement) {
-                return true;
-            }
-            if (statement instanceof PbsAst.IfStatement ifStatement) {
-                if (containsReturnInIfStatement(ifStatement)) {
-                    return true;
-                }
-                continue;
-            }
-            if (statement instanceof PbsAst.ForStatement forStatement && containsReturnStatement(forStatement.body())) {
-                return true;
-            }
-            if (statement instanceof PbsAst.WhileStatement whileStatement && containsReturnStatement(whileStatement.body())) {
-                return true;
-            }
-            if (statement instanceof PbsAst.ExpressionStatement expressionStatement && containsReturnExpression(expressionStatement.expression())) {
-                return true;
-            }
-        }
-
-        return block.tailExpression() != null && containsReturnExpression(block.tailExpression());
-    }
-
-    private boolean containsReturnInIfStatement(final PbsAst.IfStatement ifStatement) {
-        if (ifStatement == null) {
-            return false;
-        }
-        if (containsReturnStatement(ifStatement.thenBlock()) || containsReturnStatement(ifStatement.elseBlock())) {
-            return true;
-        }
-        return containsReturnInIfStatement(ifStatement.elseIf());
-    }
-
-    private boolean containsReturnExpression(final PbsAst.Expression expression) {
-        if (expression == null) {
-            return false;
-        }
-        if (expression instanceof PbsAst.BlockExpr blockExpr) {
-            return containsReturnStatement(blockExpr.block());
-        }
-        if (expression instanceof PbsAst.IfExpr ifExpr) {
-            return containsReturnStatement(ifExpr.thenBlock()) || containsReturnExpression(ifExpr.elseExpression());
-        }
-        if (expression instanceof PbsAst.SwitchExpr switchExpr) {
-            for (final var arm : switchExpr.arms()) {
-                if (containsReturnStatement(arm.block())) {
-                    return true;
-                }
-            }
-        }
-        if (expression instanceof PbsAst.HandleExpr handleExpr) {
-            for (final var arm : handleExpr.arms()) {
-                if (containsReturnStatement(arm.block())) {
-                    return true;
-                }
-            }
-        }
-        return false;
-    }
-
-    private String callableShapeKey(
-            final ReadOnlyList<PbsAst.Parameter> parameters,
-            final PbsAst.ReturnKind returnKind,
-            final PbsAst.TypeRef returnType,
-            final PbsAst.TypeRef resultErrorType) {
-        final var builder = new StringBuilder();
-        builder.append('(');
-        for (int i = 0; i < parameters.size(); i++) {
-            if (i > 0) {
-                builder.append(',');
-            }
-            builder.append(typeKey(parameters.get(i).typeRef()));
-        }
-        builder.append(")->");
-        builder.append(outputSurfaceKey(returnKind, returnType, resultErrorType));
-        return builder.toString();
-    }
-
-    private String ctorShapeKey(final ReadOnlyList<PbsAst.Parameter> parameters) {
-        final var builder = new StringBuilder();
-        builder.append('(');
-        for (int i = 0; i < parameters.size(); i++) {
-            if (i > 0) {
-                builder.append(',');
-            }
-            builder.append(typeKey(parameters.get(i).typeRef()));
-        }
-        builder.append(')');
-        return builder.toString();
-    }
-
-    private String outputSurfaceKey(
-            final PbsAst.ReturnKind returnKind,
-            final PbsAst.TypeRef returnType,
-            final PbsAst.TypeRef resultErrorType) {
-        return switch (returnKind) {
-            case INFERRED_UNIT, EXPLICIT_UNIT -> "unit";
-            case PLAIN -> carrierTypeKey(returnType);
-            case RESULT -> "result<" + typeKey(resultErrorType) + ">" + carrierTypeKey(returnType);
-        };
-    }
-
-    private String carrierTypeKey(final PbsAst.TypeRef typeRef) {
-        final var unwrapped = unwrapGroup(typeRef);
-        if (unwrapped != null && unwrapped.kind() == PbsAst.TypeRefKind.NAMED_TUPLE && unwrapped.fields().size() == 1) {
-            return typeKey(unwrapped.fields().getFirst().typeRef());
-        }
-        return typeKey(unwrapped);
-    }
-
-    private PbsAst.TypeRef unwrapGroup(final PbsAst.TypeRef typeRef) {
-        if (typeRef == null) {
-            return null;
-        }
-        if (typeRef.kind() != PbsAst.TypeRefKind.GROUP) {
-            return typeRef;
-        }
-        return unwrapGroup(typeRef.inner());
-    }
-
-    private String typeKey(final PbsAst.TypeRef typeRef) {
-        if (typeRef == null) {
-            return "unit";
-        }
-        final var unwrapped = unwrapGroup(typeRef);
-        if (unwrapped == null) {
-            return "unit";
-        }
-        return switch (unwrapped.kind()) {
-            case SIMPLE -> "simple:" + unwrapped.name();
-            case SELF -> "self";
-            case OPTIONAL -> "optional(" + typeKey(unwrapped.inner()) + ")";
-            case UNIT -> "unit";
-            case GROUP -> typeKey(unwrapped.inner());
-            case NAMED_TUPLE -> "tuple(" + unwrapped.fields().stream()
-                    .map(field -> typeKey(field.typeRef()))
-                    .reduce((left, right) -> left + "," + right)
-                    .orElse("") + ")";
-            case ERROR -> "error";
-        };
-    }
-
-    private static final class NamespaceBinder {
-        private final NameTable nameTable;
-        private final DiagnosticSink diagnostics;
-
-        private final Map<NameId, Span> typeNamespace = new HashMap<>();
-        private final Map<NameId, Span> valueNamespace = new HashMap<>();
-        private final Map<NameId, Span> hostOwnerNamespace = new HashMap<>();
-        private final Map<CallableIdentity, Span> callableNamespace = new HashMap<>();
-
-        private NamespaceBinder(final NameTable nameTable, final DiagnosticSink diagnostics) {
-            this.nameTable = nameTable;
-            this.diagnostics = diagnostics;
-        }
-
-        private void registerType(final String name, final Span span, final String kind) {
-            registerByNamespace(typeNamespace, name, span, "type namespace", kind);
-        }
-
-        private void registerValue(final String name, final Span span, final String kind) {
-            registerByNamespace(valueNamespace, name, span, "value namespace", kind);
-        }
-
-        @SuppressWarnings("unused")
-        private void registerHostOwner(final String name, final Span span, final String kind) {
-            registerByNamespace(hostOwnerNamespace, name, span, "host-owner namespace", kind);
-        }
-
-        private void registerCallable(
-                final CallableScope scope,
-                final String callableName,
-                final String shape,
-                final Span span) {
-            final var callableNameId = nameTable.register(callableName);
-            final var identity = new CallableIdentity(scope, callableNameId, shape);
-            final var first = callableNamespace.putIfAbsent(identity, span);
-            if (first == null) {
-                return;
-            }
-
-            diagnostics.report(
-                    Severity.Error,
-                    PbsSemanticsErrors.E_SEM_DUPLICATE_CALLABLE_SHAPE.name(),
-                    "Duplicate callable declaration '%s' with shape %s in %s".formatted(callableName, shape, scope.label()),
-                    span,
-                    List.of(new RelatedSpan("First callable declaration is here", first)));
-        }
-
-        private void registerCtor(
-                final CallableScope scope,
-                final String ctorName,
-                final String shape,
-                final Span span) {
-            final var callableNameId = nameTable.register(ctorName);
-            final var identity = new CallableIdentity(scope, callableNameId, shape);
-            final var first = callableNamespace.putIfAbsent(identity, span);
-            if (first == null) {
-                return;
-            }
-
-            diagnostics.report(
-                    Severity.Error,
-                    PbsSemanticsErrors.E_SEM_DUPLICATE_CALLABLE_SHAPE.name(),
-                    "Duplicate constructor declaration '%s' with shape %s in %s".formatted(ctorName, shape, scope.label()),
-                    span,
-                    List.of(new RelatedSpan("First constructor declaration is here", first)));
-        }
-
-        private void registerByNamespace(
-                final Map<NameId, Span> namespace,
-                final String declarationName,
-                final Span span,
-                final String namespaceName,
-                final String declarationKind) {
-            final var nameId = nameTable.register(declarationName);
-            final var first = namespace.putIfAbsent(nameId, span);
-            if (first == null) {
-                return;
-            }
-
-            diagnostics.report(
-                    Severity.Error,
-                    PbsSemanticsErrors.E_SEM_DUPLICATE_DECLARATION.name(),
-                    "Duplicate %s declaration '%s' in %s".formatted(declarationKind, declarationName, namespaceName),
-                    span,
-                    List.of(new RelatedSpan("First declaration is here", first)));
-        }
-    }
-
-    private record CallableIdentity(
-            CallableScope scope,
-            NameId callableNameId,
-            String shape) {
-    }
-
-    private record CallableScope(
-            String label,
-            String scopeKind,
-            NameId ownerNameId) {
-
-        private static CallableScope topLevelFunctions() {
-            return new CallableScope("top-level callable scope", "top-level", null);
-        }
-
-        private static CallableScope structMethods(final NameId ownerNameId) {
-            return new CallableScope("struct method scope", "struct", ownerNameId);
-        }
-
-        private static CallableScope structCtors(final NameId ownerNameId) {
-            return new CallableScope("struct constructor scope", "ctor", ownerNameId);
-        }
-
-        private static CallableScope serviceMethods(final NameId ownerNameId) {
-            return new CallableScope("service method scope", "service", ownerNameId);
-        }
-
-        private static CallableScope contractSignatures(final NameId ownerNameId) {
-            return new CallableScope("contract signature scope", "contract", ownerNameId);
-        }
-
-        private static CallableScope implementsMethods(final NameId ownerNameId) {
-            return new CallableScope("implements method scope", "implements", ownerNameId);
-        }
-    }
 }

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsNamespaceBinder.java

@@ -0,0 +1,106 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.source.Span;
+import p.studio.compiler.source.diagnostics.DiagnosticSink;
+import p.studio.compiler.source.diagnostics.RelatedSpan;
+import p.studio.compiler.source.diagnostics.Severity;
+import p.studio.compiler.source.identifiers.NameId;
+import p.studio.compiler.source.tables.NameTable;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+final class PbsNamespaceBinder {
+    private final NameTable nameTable;
+    private final DiagnosticSink diagnostics;
+
+    private final Map<NameId, Span> typeNamespace = new HashMap<>();
+    private final Map<NameId, Span> valueNamespace = new HashMap<>();
+    private final Map<NameId, Span> hostOwnerNamespace = new HashMap<>();
+    private final Map<CallableIdentity, Span> callableNamespace = new HashMap<>();
+
+    PbsNamespaceBinder(final NameTable nameTable, final DiagnosticSink diagnostics) {
+        this.nameTable = nameTable;
+        this.diagnostics = diagnostics;
+    }
+
+    void registerType(final String name, final Span span, final String kind) {
+        registerByNamespace(typeNamespace, name, span, "type namespace", kind);
+    }
+
+    void registerValue(final String name, final Span span, final String kind) {
+        registerByNamespace(valueNamespace, name, span, "value namespace", kind);
+    }
+
+    @SuppressWarnings("unused")
+    void registerHostOwner(final String name, final Span span, final String kind) {
+        registerByNamespace(hostOwnerNamespace, name, span, "host-owner namespace", kind);
+    }
+
+    void registerCallable(
+            final PbsCallableScope scope,
+            final String callableName,
+            final String shape,
+            final Span span) {
+        final var callableNameId = nameTable.register(callableName);
+        final var identity = new CallableIdentity(scope, callableNameId, shape);
+        final var first = callableNamespace.putIfAbsent(identity, span);
+        if (first == null) {
+            return;
+        }
+
+        diagnostics.report(
+                Severity.Error,
+                PbsSemanticsErrors.E_SEM_DUPLICATE_CALLABLE_SHAPE.name(),
+                "Duplicate callable declaration '%s' with shape %s in %s".formatted(callableName, shape, scope.label()),
+                span,
+                List.of(new RelatedSpan("First callable declaration is here", first)));
+    }
+
+    void registerCtor(
+            final PbsCallableScope scope,
+            final String ctorName,
+            final String shape,
+            final Span span) {
+        final var callableNameId = nameTable.register(ctorName);
+        final var identity = new CallableIdentity(scope, callableNameId, shape);
+        final var first = callableNamespace.putIfAbsent(identity, span);
+        if (first == null) {
+            return;
+        }
+
+        diagnostics.report(
+                Severity.Error,
+                PbsSemanticsErrors.E_SEM_DUPLICATE_CALLABLE_SHAPE.name(),
+                "Duplicate constructor declaration '%s' with shape %s in %s".formatted(ctorName, shape, scope.label()),
+                span,
+                List.of(new RelatedSpan("First constructor declaration is here", first)));
+    }
+
+    private void registerByNamespace(
+            final Map<NameId, Span> namespace,
+            final String declarationName,
+            final Span span,
+            final String namespaceName,
+            final String declarationKind) {
+        final var nameId = nameTable.register(declarationName);
+        final var first = namespace.putIfAbsent(nameId, span);
+        if (first == null) {
+            return;
+        }
+
+        diagnostics.report(
+                Severity.Error,
+                PbsSemanticsErrors.E_SEM_DUPLICATE_DECLARATION.name(),
+                "Duplicate %s declaration '%s' in %s".formatted(declarationKind, declarationName, namespaceName),
+                span,
+                List.of(new RelatedSpan("First declaration is here", first)));
+    }
+
+    private record CallableIdentity(
+            PbsCallableScope scope,
+            NameId callableNameId,
+            String shape) {
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsTypeSurfaceSemanticsValidator.java

@@ -0,0 +1,59 @@
+package p.studio.compiler.pbs.semantics;
+
+import p.studio.compiler.pbs.ast.PbsAst;
+import p.studio.compiler.source.diagnostics.DiagnosticSink;
+
+import java.util.ArrayList;
+import java.util.List;
+
+final class PbsTypeSurfaceSemanticsValidator {
+    private PbsTypeSurfaceSemanticsValidator() {
+    }
+
+    static void validateTypeSurfaceRecursive(
+            final List<PbsAst.TypeRef> roots,
+            final String ownerDescription,
+            final DiagnosticSink diagnostics) {
+        final var pending = new ArrayList<>(roots);
+        while (!pending.isEmpty()) {
+            final var typeRef = pending.removeLast();
+            if (typeRef == null) {
+                continue;
+            }
+
+            switch (typeRef.kind()) {
+                case OPTIONAL -> {
+                    if (typeRef.inner() == null || typeRef.inner().kind() == PbsAst.TypeRefKind.UNIT) {
+                        diagnostics.error(
+                                PbsSemanticsErrors.E_SEM_INVALID_OPTIONAL_VOID_TYPE_SURFACE.name(),
+                                "Invalid optional type surface in %s: 'optional void' is not allowed".formatted(ownerDescription),
+                                typeRef.span());
+                    }
+                    pending.add(typeRef.inner());
+                }
+                case GROUP -> pending.add(typeRef.inner());
+                case NAMED_TUPLE -> {
+                    for (final var field : typeRef.fields()) {
+                        pending.add(field.typeRef());
+                    }
+                }
+                default -> {
+                    // No recursive children.
+                }
+            }
+        }
+    }
+
+    static boolean isOptionalType(final PbsAst.TypeRef typeRef) {
+        if (typeRef == null) {
+            return false;
+        }
+        if (typeRef.kind() == PbsAst.TypeRefKind.OPTIONAL) {
+            return true;
+        }
+        if (typeRef.kind() == PbsAst.TypeRefKind.GROUP) {
+            return isOptionalType(typeRef.inner());
+        }
+        return false;
+    }
+}