# PBS AST Workshop 4

Status: Active

## Purpose

Close AST-facing diagnostics/recovery rules and Gate U evidence needed to finalize the AST contract.

## Decisions To Produce

1. Required diagnostics attribution behavior sourced from AST.
2. Recovery invariants for parser output after syntax errors.
3. Mandatory AST fixture corpus for Gate U evidence.
4. Explicit deferred list for non-AST concerns.

## Candidate Decisions

### 1. AST Must Sustain Diagnostics Fidelity

- Primary attribution for syntax diagnostics must remain stable and source-anchored.
- Cross-file related spans are attached when required by diagnostics rules.

### 2. Recovery Must Preserve Structural Integrity

- Recovered AST may be partial, but must remain structurally coherent.
- Recovery must not fabricate semantically valid shapes that hide syntax failure.

### 3. Gate U Evidence Baseline

- AST conformance evidence includes representative valid trees, invalid/recovery cases, and attribution assertions.

## Questions To Resolve

1. Which recovery behaviors are mandatory versus implementation-defined?
2. What is the minimum fixture set to prevent AST regressions?
3. Which AST-facing diagnostics codes must be frozen now?

## Expected Outputs

1. Decision note on AST diagnostics/recovery policy.
2. Decision note on Gate U AST evidence baseline.
3. Closure checklist for `11. AST Specification.md`.

## Explicit Deferrals

- IRBackend lowering details (owned by `13`),
- backend/runtime/verifier concerns,
- compatibility/publication policy.

## Inputs

- `docs/pbs/specs/11. AST Specification.md`
- `docs/pbs/specs/12. Diagnostics Specification.md`
- `docs/general/specs/13. Conformance Test Specification.md`
- `docs/pbs/agendas/11.3. AST Workshop 3 - Statements and Expressions.md`