implements PLN-0007

discussion/index.ndjson

@@ -4,6 +4,6 @@
 {"type":"discussion","id":"DSC-0003","status":"done","ticket":"packer-docs-import","title":"Import docs/packer into discussion-framework artifacts","created_at":"2026-03-26","updated_at":"2026-03-26","tags":["packer","migration","discussion-framework","docs-import"],"agendas":[],"decisions":[],"plans":[],"lessons":[{"id":"LSN-0009","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0009-mental-model-packer-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0010","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0010-asset-identity-and-runtime-contract-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0011","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0011-foundations-workspace-runtime-and-build-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0012","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0012-runtime-ownership-and-studio-boundary-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0013","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0013-metadata-convergence-and-runtime-sink-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0014","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0014-pack-wizard-summary-validation-and-pack-execution-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0015","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0015-tile-bank-packing-contract-legacy.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"},{"id":"LSN-0017","file":"discussion/lessons/DSC-0003-packer-docs-import/LSN-0017-packer-docs-import-pattern.md","status":"done","created_at":"2026-03-26","updated_at":"2026-03-26"}]}
 {"type":"discussion","id":"DSC-0004","status":"open","ticket":"tilemap-and-metatile-runtime-binary-layout","title":"Tilemap and Metatile Runtime Binary Layout","created_at":"2026-03-26","updated_at":"2026-03-26","tags":["packer","legacy-import","tilemap","metatile","runtime-layout"],"agendas":[{"id":"AGD-0004","file":"AGD-0004-tilemap-and-metatile-runtime-binary-layout.md","status":"open","created_at":"2026-03-26","updated_at":"2026-03-26"}],"decisions":[],"plans":[],"lessons":[]}
 {"type":"discussion","id":"DSC-0005","status":"open","ticket":"variable-tile-bank-palette-serialization","title":"Variable Tile Bank Palette Serialization","created_at":"2026-03-26","updated_at":"2026-03-26","tags":["packer","legacy-import","tile-bank","palette-serialization","versioning"],"agendas":[{"id":"AGD-0005","file":"AGD-0005-variable-tile-bank-palette-serialization.md","status":"open","created_at":"2026-03-26","updated_at":"2026-03-26"}],"decisions":[],"plans":[],"lessons":[]}
-{"type":"discussion","id":"DSC-0006","status":"open","ticket":"pbs-game-facing-asset-refs-and-call-result-discard","title":"PBS Game-Facing Asset References and Ignored Call Result Lowering","created_at":"2026-03-27","updated_at":"2026-03-27","tags":["compiler","pbs","ergonomics","lowering","runtime","asset-identity","expression-statements"],"agendas":[{"id":"AGD-0006","file":"AGD-0006-pbs-game-facing-asset-refs-and-call-result-discard.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27"}],"decisions":[{"id":"DEC-0005","file":"DEC-0005-pbs-asset-address-surface-and-be-lowering.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27","ref_agenda":"AGD-0006"},{"id":"DEC-0006","file":"DEC-0006-pbs-ignored-values-lowering-and-warning.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27","ref_agenda":"AGD-0006"}],"plans":[{"id":"PLN-0005","file":"PLN-0005-pbs-asset-address-surface-spec-propagation.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0005"]},{"id":"PLN-0006","file":"PLN-0006-pbs-asset-address-surface-implementation.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0005"]},{"id":"PLN-0007","file":"PLN-0007-pbs-ignored-values-warning-implementation.md","status":"review","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0006"]}],"lessons":[]}
+{"type":"discussion","id":"DSC-0006","status":"open","ticket":"pbs-game-facing-asset-refs-and-call-result-discard","title":"PBS Game-Facing Asset References and Ignored Call Result Lowering","created_at":"2026-03-27","updated_at":"2026-03-27","tags":["compiler","pbs","ergonomics","lowering","runtime","asset-identity","expression-statements"],"agendas":[{"id":"AGD-0006","file":"AGD-0006-pbs-game-facing-asset-refs-and-call-result-discard.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27"}],"decisions":[{"id":"DEC-0005","file":"DEC-0005-pbs-asset-address-surface-and-be-lowering.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27","ref_agenda":"AGD-0006"},{"id":"DEC-0006","file":"DEC-0006-pbs-ignored-values-lowering-and-warning.md","status":"accepted","created_at":"2026-03-27","updated_at":"2026-03-27","ref_agenda":"AGD-0006"}],"plans":[{"id":"PLN-0005","file":"PLN-0005-pbs-asset-address-surface-spec-propagation.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0005"]},{"id":"PLN-0006","file":"PLN-0006-pbs-asset-address-surface-implementation.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0005"]},{"id":"PLN-0007","file":"PLN-0007-pbs-ignored-values-warning-implementation.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27","ref_decisions":["DEC-0006"]}],"lessons":[]}
 {"type":"discussion","id":"DSC-0007","status":"done","ticket":"pbs-learn-to-discussion-lessons-migration","title":"Migrate PBS Learn Documents into Discussion Lessons","created_at":"2026-03-27","updated_at":"2026-03-27","tags":["compiler","pbs","migration","discussion-framework","lessons","learn-prune"],"agendas":[],"decisions":[],"plans":[],"lessons":[{"id":"LSN-0018","file":"discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0018-pbs-ast-and-parser-contract-legacy.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"},{"id":"LSN-0019","file":"discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0019-pbs-name-resolution-and-linking-legacy.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"},{"id":"LSN-0020","file":"discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0020-pbs-runtime-values-identity-memory-boundaries-legacy.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"},{"id":"LSN-0021","file":"discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0021-pbs-diagnostics-and-conformance-governance-legacy.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"},{"id":"LSN-0022","file":"discussion/lessons/DSC-0007-pbs-learn-to-discussion-lessons-migration/LSN-0022-pbs-globals-lifecycle-and-published-entrypoint-legacy.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"}]}
 {"type":"discussion","id":"DSC-0008","status":"done","ticket":"pbs-low-level-asset-manager-surface","title":"PBS Low-Level Asset Manager Surface for Runtime AssetManager","created_at":"2026-03-27","updated_at":"2026-03-27","tags":["compiler","pbs","runtime","asset-manager","host-abi","stdlib","asset"],"agendas":[],"decisions":[],"plans":[],"lessons":[{"id":"LSN-0023","file":"discussion/lessons/DSC-0008-pbs-low-level-asset-manager-surface/LSN-0023-lowassets-runtime-aligned-sdk-surface.md","status":"done","created_at":"2026-03-27","updated_at":"2026-03-27"}]}

discussion/workflow/plans/PLN-0007-pbs-ignored-values-warning-implementation.md

@@ -2,9 +2,9 @@
 id: PLN-0007
 ticket: pbs-game-facing-asset-refs-and-call-result-discard
 title: Implement DEC-0006 Ignored Values Lowering and Warning
-status: review
+status: done
 created: 2026-03-27
-completed:
+completed: 2026-03-27
 tags: [compiler, pbs, implementation, diagnostics, lowering, expression-statements, warnings]
 ---
 

docs/specs/compiler-languages/pbs/12. Diagnostics Specification.md

@@ -203,6 +203,9 @@ At minimum, the PBS diagnostics baseline must cover:
    - unresolved terminal asset reference,
    - namespace-versus-terminal asset misuse,
    - structurally invalid symbolic asset path admitted by source syntax but rejected by semantic surface rules.
+10. ignored-value diagnostics for expression statements, including:
+   - a stable warning when a materialized value is produced and then ignored,
+   - and no warning for unit-like expression statements.
 
 At minimum, host-admission diagnostics must cover missing or malformed host capability metadata and unknown or undeclared capability names.
 
@@ -229,7 +232,9 @@ When failures exist, implementations must still preserve deterministic diagnosti
 
 ## 11. Cost Diagnostics and Warning Policy
 
-Warnings are supported and recommended in v1, but they are not part of the minimum mandatory conformance baseline.
+Warnings are supported in v1, and one warning is mandatory for PBS core: ignored materialized values in expression statements.
+
+The mandatory warning identity for this rule is `W_SEM_IGNORED_VALUE`.
 
 Tooling may surface qualitative cost facts as warnings or notes, especially for:
 

docs/specs/compiler-languages/pbs/13. Lowering IRBackend Specification.md

@@ -188,6 +188,12 @@ When a host-backed callsite also depends on backend-owned symbolic asset lowerin
 - the preserved symbolic operand MUST remain attributable to a backend-provided `Addressable` identity;
 - backend stages MUST be able to rewrite that symbolic operand into runtime-facing `asset_id` before the final low-level host path is emitted.
 
+When an expression statement leaves a materialized runtime value on the stack:
+
+- `IRBackend` lowering MUST emit explicit discard behavior rather than relying on implicit backend stack cleanup;
+- for single-slot materialized values this discard is represented by `POP`;
+- and the discard path MUST preserve stack-accounting validity for subsequent backend lowering stages.
+
 ### 12.4 VM-owned metadata obligations
 
 VM-owned intrinsic callsites admitted at this boundary must preserve canonical intrinsic identity:

docs/specs/compiler-languages/pbs/4. Static Semantics Specification.md

@@ -238,6 +238,9 @@ Rules:
 - Symbolic asset references admitted by the frontend MUST resolve against the backend-provided `Addressable` surface rather than by direct packer queries.
 - A valid symbolic asset reference MUST correspond to a terminal `Addressable` entry in the backend-provided surface.
 - Intermediate `assets...` namespace nodes MUST NOT type-check as `Addressable` values.
+- An expression statement MAY evaluate an expression that has a materialized value type.
+- When an expression statement ignores a materialized value, PBS v1 MUST emit the ignored-value warning defined by the diagnostics specification.
+- Unit-like expression statements MUST NOT emit the ignored-value warning.
 
 ### 3.4 Constant expressions
 

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/lowering/PbsExecutableBodyLowerer.java

@@ -143,6 +143,7 @@ final class PbsExecutableBodyLowerer {
             case PbsAst.WhileStatement whileStatement -> lowerWhileStatement(whileStatement, context);
             case PbsAst.ExpressionStatement expressionStatement -> {
                 lowerExpression(expressionStatement.expression(), context);
+                discardIgnoredValueSlots(expressionStatement.expression(), context);
                 yield false;
             }
             case PbsAst.BreakStatement breakStatement -> lowerBreakStatement(breakStatement, context);
@@ -426,6 +427,21 @@ final class PbsExecutableBodyLowerer {
         }
     }
 
+    private void discardIgnoredValueSlots(
+            final PbsAst.Expression expression,
+            final PbsExecutableLoweringContext context) {
+        final var slots = materializedValueSlots(expression, context);
+        for (int i = 0; i < slots; i++) {
+            context.instructions().add(new IRBackendExecutableFunction.Instruction(
+                    IRBackendExecutableFunction.InstructionKind.POP,
+                    "",
+                    null,
+                    null,
+                    null,
+                    expression == null ? p.studio.compiler.source.Span.none() : expression.span()));
+        }
+    }
+
     private void lowerMemberExpression(
             final PbsAst.MemberExpr memberExpr,
             final PbsExecutableLoweringContext context) {
@@ -508,6 +524,92 @@ final class PbsExecutableBodyLowerer {
         return receiverPath + "." + memberExpr.memberName();
     }
 
+    private int materializedValueSlots(
+            final PbsAst.Expression expression,
+            final PbsExecutableLoweringContext context) {
+        if (expression == null) {
+            return 0;
+        }
+        return switch (expression) {
+            case PbsAst.CallExpr callExpr -> callsiteEmitter.returnSlotsOf(callExpr, context);
+            case PbsAst.ApplyExpr ignored -> 0;
+            case PbsAst.BinaryExpr ignored -> 1;
+            case PbsAst.UnaryExpr ignored -> 1;
+            case PbsAst.ElseExpr elseExpr ->
+                    Math.max(
+                            materializedValueSlots(elseExpr.optionalExpression(), context),
+                            materializedValueSlots(elseExpr.fallbackExpression(), context));
+            case PbsAst.IfExpr ifExpr ->
+                    Math.max(
+                            blockValueSlots(ifExpr.thenBlock(), context),
+                            materializedValueSlots(ifExpr.elseExpression(), context));
+            case PbsAst.SwitchExpr ignored -> 0;
+            case PbsAst.HandleExpr ignored -> 0;
+            case PbsAst.AsExpr asExpr -> materializedValueSlots(asExpr.expression(), context);
+            case PbsAst.MemberExpr memberExpr -> {
+                final var assetReference = flattenAssetReference(memberExpr);
+                if (assetReference != null && context.resolveAssetId(assetReference) != null) {
+                    yield 1;
+                }
+                yield materializedValueSlots(memberExpr.receiver(), context);
+            }
+            case PbsAst.PropagateExpr propagateExpr -> materializedValueSlots(propagateExpr.expression(), context);
+            case PbsAst.GroupExpr groupExpr -> materializedValueSlots(groupExpr.expression(), context);
+            case PbsAst.NewExpr newExpr -> expressionListValueSlots(newExpr.arguments(), context);
+            case PbsAst.BindExpr bindExpr -> materializedValueSlots(bindExpr.contextExpression(), context);
+            case PbsAst.SomeExpr someExpr -> materializedValueSlots(someExpr.value(), context);
+            case PbsAst.OkExpr okExpr -> materializedValueSlots(okExpr.value(), context);
+            case PbsAst.TupleExpr tupleExpr -> {
+                var slots = 0;
+                for (final var item : tupleExpr.items()) {
+                    slots += materializedValueSlots(item.expression(), context);
+                }
+                yield slots;
+            }
+            case PbsAst.BlockExpr blockExpr -> blockValueSlots(blockExpr.block(), context);
+            case PbsAst.IdentifierExpr identifierExpr -> producesIdentifierValue(identifierExpr, context) ? 1 : 0;
+            case PbsAst.IntLiteralExpr ignored -> 1;
+            case PbsAst.StringLiteralExpr ignoredString -> 1;
+            case PbsAst.BoolLiteralExpr ignoredBool -> 1;
+            case PbsAst.FloatLiteralExpr ignoredFloat -> 0;
+            case PbsAst.BoundedLiteralExpr ignoredBounded -> 0;
+            case PbsAst.ThisExpr ignoredThis -> 0;
+            case PbsAst.NoneExpr ignoredNone -> 0;
+            case PbsAst.ErrExpr ignoredErr -> 0;
+            case PbsAst.UnitExpr ignoredUnit -> 0;
+        };
+    }
+
+    private int blockValueSlots(
+            final PbsAst.Block block,
+            final PbsExecutableLoweringContext context) {
+        if (block == null || block.tailExpression() == null) {
+            return 0;
+        }
+        return materializedValueSlots(block.tailExpression(), context);
+    }
+
+    private int expressionListValueSlots(
+            final ReadOnlyList<PbsAst.Expression> expressions,
+            final PbsExecutableLoweringContext context) {
+        var slots = 0;
+        for (final var expression : expressions) {
+            slots += materializedValueSlots(expression, context);
+        }
+        return slots;
+    }
+
+    private boolean producesIdentifierValue(
+            final PbsAst.IdentifierExpr identifierExpr,
+            final PbsExecutableLoweringContext context) {
+        if ("assets".equals(identifierExpr.name())) {
+            return false;
+        }
+        return context.resolveLocalSlot(identifierExpr.name()) != null
+                || context.resolveGlobalSlot(identifierExpr.name()) != null
+                || context.resolveConstDecl(identifierExpr.name()) != null;
+    }
+
     private void lowerConstExpression(
             final PbsAst.Expression expression,
             final PbsExecutableLoweringContext context) {

prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/lowering/PbsExecutableCallsiteEmitter.java

@@ -16,6 +16,29 @@ import java.util.Map;
 import static p.studio.compiler.pbs.lowering.PbsExecutableLoweringModels.*;
 
 final class PbsExecutableCallsiteEmitter {
+    int returnSlotsOf(
+            final PbsAst.CallExpr callExpr,
+            final PbsExecutableLoweringContext context) {
+        final var calleeIdentity = resolveCalleeIdentity(callExpr.callee(), context.nameTable());
+        if (calleeIdentity == null) {
+            return 0;
+        }
+        final var candidates = resolveCallsiteCandidates(callExpr, calleeIdentity, context);
+        final var resolvedCategory = resolveCallsiteCategory(candidates, callExpr, context);
+        if (resolvedCategory == null) {
+            return 0;
+        }
+        return switch (resolvedCategory) {
+            case HOST -> candidates.hostCandidates().getFirst().retSlots();
+            case INTRINSIC -> candidates.intrinsicCandidates().getFirst().retSlots();
+            case CALLABLE -> {
+                if (candidates.callableCandidates().size() != 1) {
+                    yield 0;
+                }
+                yield context.returnSlotsByCallableId().getOrDefault(candidates.callableCandidates().getFirst(), 0);
+            }
+        };
+    }
 
     void emitCallsite(
             final PbsAst.CallExpr callExpr,

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

@@ -172,7 +172,7 @@ final class PbsFlowStatementAnalyzer {
             return;
         }
         if (statement instanceof PbsAst.ExpressionStatement expressionStatement) {
-            expressionAnalyzer.analyzeExpression(
+            final var expressionType = expressionAnalyzer.analyzeExpression(
                     expressionStatement.expression(),
                     scope,
                     null,
@@ -183,7 +183,13 @@ final class PbsFlowStatementAnalyzer {
                     diagnostics,
                     ExprUse.VALUE,
                     false,
-                    this::analyzeNestedBlock);
+                    this::analyzeNestedBlock).type();
+            if (typeOps.isMaterializedValue(expressionType)) {
+                Diagnostics.warning(diagnostics,
+                        PbsSemanticsErrors.W_SEM_IGNORED_VALUE.name(),
+                        "Ignored materialized value in expression statement",
+                        expressionStatement.span());
+            }
             return;
         }
         if (statement instanceof PbsAst.AssignStatement assignStatement) {

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

@@ -139,6 +139,16 @@ final class PbsFlowTypeOps {
         return type.kind() == Kind.STR || type.kind() == Kind.UNKNOWN;
     }
 
+    boolean isMaterializedValue(final TypeView type) {
+        if (type == null) {
+            return false;
+        }
+        return switch (type.kind()) {
+            case UNKNOWN, UNIT, ASSET_NAMESPACE -> false;
+            default -> true;
+        };
+    }
+
     private boolean tupleCompatible(final TypeView actual, final TypeView expected) {
         if (actual.tupleFields().size() != expected.tupleFields().size()) {
             return false;

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

@@ -69,4 +69,5 @@ public enum PbsSemanticsErrors {
     E_SEM_HANDLE_ERROR_MISMATCH,
     E_SEM_HANDLE_ARM_TERMINAL_INVALID,
     E_SEM_EXEC_LOWERING_UNRESOLVED_CALLEE,
+    W_SEM_IGNORED_VALUE,
 }

prometeu-compiler/frontends/prometeu-frontend-pbs/src/test/java/p/studio/compiler/pbs/PbsDiagnosticsContractTest.java

@@ -10,6 +10,7 @@ import p.studio.compiler.pbs.parser.PbsParser;
 import p.studio.compiler.pbs.semantics.PbsSemanticsErrors;
 import p.studio.compiler.source.diagnostics.DiagnosticPhase;
 import p.studio.compiler.source.diagnostics.DiagnosticSink;
+import p.studio.compiler.source.diagnostics.Severity;
 import p.studio.compiler.source.identifiers.FileId;
 import p.studio.utilities.structures.ReadOnlyList;
 
@@ -52,6 +53,28 @@ class PbsDiagnosticsContractTest {
         assertEquals(DiagnosticPhase.STATIC_SEMANTICS, diagnostic.getPhase());
     }
 
+    @Test
+    void shouldTagIgnoredValueWarningsWithStaticSemanticsPhase() {
+        final var source = """
+                fn pure() -> int { return 1; }
+                fn frame() -> void {
+                  pure();
+                  return;
+                }
+                """;
+        final var diagnostics = DiagnosticSink.empty();
+
+        new PbsFrontendCompiler().compileFile(new FileId(11), source, diagnostics);
+
+        final var diagnostic = diagnostics.stream()
+                .filter(d -> d.getCode().equals(PbsSemanticsErrors.W_SEM_IGNORED_VALUE.name()))
+                .findFirst()
+                .orElseThrow();
+        assertEquals(DiagnosticPhase.STATIC_SEMANTICS, diagnostic.getPhase());
+        assertEquals(Severity.Warning, diagnostic.getSeverity());
+        assertEquals(diagnostic.getCode(), diagnostic.getTemplateId());
+    }
+
     @Test
     void shouldTagLinkingDiagnosticsAndExposeRelatedSpanForDuplicateBarrelEntry() {
         final var diagnostics = DiagnosticSink.empty();

prometeu-compiler/frontends/prometeu-frontend-pbs/src/test/java/p/studio/compiler/pbs/PbsFrontendCompilerTest.java

@@ -288,13 +288,17 @@ class PbsFrontendCompilerTest {
         final var compiler = new PbsFrontendCompiler();
         final var fileBackend = compiler.compileFile(new FileId(99), source, diagnostics);
 
-        assertTrue(diagnostics.isEmpty(), "Valid program should not report diagnostics");
+        assertEquals(0, diagnostics.errorCount(), "Valid program should not report errors");
+        assertEquals(2, diagnostics.warningCount());
         final var executableA = fileBackend.executableFunctions().stream()
                 .filter(fn -> fn.callableName().equals("a"))
                 .findFirst()
                 .orElseThrow();
-        assertEquals("b", executableA.instructions().get(0).calleeCallableName());
+        final var callNames = executableA.instructions().stream()
-        assertEquals("c", executableA.instructions().get(1).calleeCallableName());
+                .filter(i -> i.kind() == p.studio.compiler.models.IRBackendExecutableFunction.InstructionKind.CALL_FUNC)
+                .map(p.studio.compiler.models.IRBackendExecutableFunction.Instruction::calleeCallableName)
+                .toList();
+        assertEquals(java.util.List.of("b", "c"), callNames);
     }
 
     @Test
@@ -312,7 +316,8 @@ class PbsFrontendCompilerTest {
         final var compiler = new PbsFrontendCompiler();
         final var fileBackend = compiler.compileFile(new FileId(100), source, diagnostics);
 
-        assertTrue(diagnostics.isEmpty(), "Valid program should not report diagnostics");
+        assertEquals(0, diagnostics.errorCount(), "Valid program should not report errors");
+        assertEquals(1, diagnostics.warningCount());
         final var executableMain = fileBackend.executableFunctions().stream()
                 .filter(fn -> fn.callableName().equals("main"))
                 .findFirst()
@@ -512,6 +517,61 @@ class PbsFrontendCompilerTest {
         assertEquals(0, fileBackend.executableFunctions().size());
     }
 
+    @Test
+    void shouldWarnAndDiscardIgnoredMaterializedExpressionStatements() {
+        final var source = """
+                fn pure() -> int { return 1; }
+
+                fn frame() -> void {
+                  pure();
+                  1 + 2;
+                  return;
+                }
+                """;
+
+        final var diagnostics = DiagnosticSink.empty();
+        final var compiler = new PbsFrontendCompiler();
+        final var fileBackend = compiler.compileFile(new FileId(14), source, diagnostics);
+
+        assertEquals(2, diagnostics.warningCount());
+        assertTrue(diagnostics.stream().anyMatch(d ->
+                d.getCode().equals(PbsSemanticsErrors.W_SEM_IGNORED_VALUE.name())));
+        final var executableFrame = fileBackend.executableFunctions().stream()
+                .filter(fn -> fn.callableName().equals("frame"))
+                .findFirst()
+                .orElseThrow();
+        assertTrue(executableFrame.instructions().stream().anyMatch(i ->
+                i.kind() == p.studio.compiler.models.IRBackendExecutableFunction.InstructionKind.CALL_FUNC));
+        assertEquals(2, executableFrame.instructions().stream()
+                .filter(i -> i.kind() == p.studio.compiler.models.IRBackendExecutableFunction.InstructionKind.POP)
+                .count());
+    }
+
+    @Test
+    void shouldNotWarnOrDiscardUnitExpressionStatements() {
+        final var source = """
+                fn log() -> void { return; }
+
+                fn frame() -> void {
+                  log();
+                  return;
+                }
+                """;
+
+        final var diagnostics = DiagnosticSink.empty();
+        final var compiler = new PbsFrontendCompiler();
+        final var fileBackend = compiler.compileFile(new FileId(15), source, diagnostics);
+
+        assertEquals(0, diagnostics.warningCount());
+        final var executableFrame = fileBackend.executableFunctions().stream()
+                .filter(fn -> fn.callableName().equals("frame"))
+                .findFirst()
+                .orElseThrow();
+        assertEquals(0, executableFrame.instructions().stream()
+                .filter(i -> i.kind() == p.studio.compiler.models.IRBackendExecutableFunction.InstructionKind.POP)
+                .count());
+    }
+
     @Test
     void shouldRejectHostBindingWithoutCapabilityAtHostAdmission() {
         final var source = """

prometeu-compiler/frontends/prometeu-frontend-pbs/src/test/java/p/studio/compiler/pbs/semantics/PbsIgnoredValueSemanticsTest.java

@@ -0,0 +1,48 @@
+package p.studio.compiler.pbs.semantics;
+
+import org.junit.jupiter.api.Test;
+import p.studio.compiler.pbs.PbsFrontendCompiler;
+import p.studio.compiler.source.diagnostics.DiagnosticSink;
+import p.studio.compiler.source.identifiers.FileId;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class PbsIgnoredValueSemanticsTest {
+
+    @Test
+    void shouldWarnWhenExpressionStatementIgnoresMaterializedValue() {
+        final var source = """
+                fn pure() -> int { return 1; }
+
+                fn frame() -> void {
+                  pure();
+                  return;
+                }
+                """;
+        final var diagnostics = DiagnosticSink.empty();
+
+        new PbsFrontendCompiler().compileFile(new FileId(0), source, diagnostics);
+
+        assertEquals(1, diagnostics.warningCount());
+        assertTrue(diagnostics.stream().anyMatch(d ->
+                d.getCode().equals(PbsSemanticsErrors.W_SEM_IGNORED_VALUE.name())));
+    }
+
+    @Test
+    void shouldNotWarnWhenExpressionStatementIsUnitLike() {
+        final var source = """
+                fn log() -> void { return; }
+
+                fn frame() -> void {
+                  log();
+                  return;
+                }
+                """;
+        final var diagnostics = DiagnosticSink.empty();
+
+        new PbsFrontendCompiler().compileFile(new FileId(1), source, diagnostics);
+
+        assertEquals(0, diagnostics.warningCount());
+    }
+}

prometeu-compiler/frontends/prometeu-frontend-pbs/src/test/java/p/studio/compiler/services/PBSFrontendPhaseServiceTest.java

@@ -87,7 +87,8 @@ class PBSFrontendPhaseServiceTest {
                 LogAggregator.empty(),
                 BuildingIssueSink.empty());
 
-        assertTrue(diagnostics.isEmpty(), diagnostics.stream().map(d -> d.getCode() + ":" + d.getMessage()).toList().toString());
+        assertEquals(0, diagnostics.errorCount(), diagnostics.stream().map(d -> d.getCode() + ":" + d.getMessage()).toList().toString());
+        assertEquals(0, diagnostics.warningCount());
         final var frameFn = irBackend.getExecutableFunctions().stream()
                 .filter(function -> "frame".equals(function.callableName()))
                 .findFirst()
@@ -97,6 +98,62 @@ class PBSFrontendPhaseServiceTest {
                         && "91".equals(instruction.label())));
     }
 
+    @Test
+    void shouldKeepWarningsNonBlockingWhileLoweringExplicitDiscard() throws IOException {
+        final var projectRoot = tempDir.resolve("project-ignored-values");
+        final var sourceRoot = projectRoot.resolve("src");
+        final var modulePath = sourceRoot.resolve("main");
+        Files.createDirectories(modulePath);
+
+        final var sourceFile = modulePath.resolve("source.pbs");
+        final var modBarrel = modulePath.resolve("mod.barrel");
+        Files.writeString(sourceFile, """
+                fn pure() -> int { return 1; }
+
+                [Frame]
+                fn frame() -> void {
+                  pure();
+                  return;
+                }
+                """);
+        Files.writeString(modBarrel, "pub fn frame() -> void;");
+
+        final var projectTable = new ProjectTable();
+        final var fileTable = new FileTable(1);
+        final var projectId = projectTable.register(ProjectDescriptor.builder()
+                .rootPath(projectRoot)
+                .name("core")
+                .version("1.0.0")
+                .sourceRoots(ReadOnlyList.wrap(List.of(sourceRoot)))
+                .build());
+
+        registerFile(projectId, projectRoot, sourceFile, fileTable);
+        registerFile(projectId, projectRoot, modBarrel, fileTable);
+
+        final var ctx = new FrontendPhaseContext(
+                projectTable,
+                fileTable,
+                new BuildStack(ReadOnlyList.wrap(List.of(projectId))));
+        final var diagnostics = DiagnosticSink.empty();
+
+        final var irBackend = new PBSFrontendPhaseService().compile(
+                ctx,
+                diagnostics,
+                LogAggregator.empty(),
+                BuildingIssueSink.empty());
+
+        assertEquals(1, diagnostics.warningCount());
+        assertEquals(0, diagnostics.errorCount());
+        assertTrue(diagnostics.stream().anyMatch(d ->
+                d.getCode().equals(PbsSemanticsErrors.W_SEM_IGNORED_VALUE.name())));
+        final var frameFn = irBackend.getExecutableFunctions().stream()
+                .filter(function -> "frame".equals(function.callableName()))
+                .findFirst()
+                .orElseThrow();
+        assertTrue(frameFn.instructions().stream().anyMatch(instruction ->
+                instruction.kind() == p.studio.compiler.models.IRBackendExecutableFunction.InstructionKind.POP));
+    }
+
     @Test
     void shouldReportInvalidBarrelFilename() throws IOException {
         final var projectRoot = tempDir.resolve("project");
@@ -259,7 +316,8 @@ class PBSFrontendPhaseServiceTest {
                 LogAggregator.empty(),
                 BuildingIssueSink.empty());
 
-        assertTrue(diagnostics.isEmpty(), diagnostics.stream().map(d -> d.getCode() + ":" + d.getMessage()).toList().toString());
+        assertEquals(0, diagnostics.errorCount(), diagnostics.stream().map(d -> d.getCode() + ":" + d.getMessage()).toList().toString());
+        assertEquals(1, diagnostics.warningCount());
 
         final var targetFn = irBackend.getExecutableFunctions().stream()
                 .filter(function -> "target".equals(function.callableName()))