# PBS Frontend IR and Lowering Workshop 4

Status: Active

## Purpose

Run the fourth focused discussion for `13. Lowering IRBackend Specification.md` to close frontend-specific pending points:

- builtin-facing source semantics at frontend lowering boundary,
- diagnostics stability for lowering-related failures,
- and test-gate evidence needed to close v1 frontend slice.

## Why This Slice Last

This workshop should close only after Workshops 1 to 3 define the base lowering contract and callable status map.

## Proposed Meeting Order

1. Reconfirm frontend-only scope and deferred backend concerns.
2. Close builtin-shell obligations at frontend lowering boundary.
3. Close diagnostics identity/attribution obligations for lowering-related failures.
4. Close minimum gate evidence package (Gate U).
5. Produce closure checklist for spec `13` frontend slice.

## Decisions To Produce

1. Frontend-lowering obligations for builtin-facing constructs in v1.
2. Stable diagnostics contract for frontend-lowering rejections.
3. Mandatory fixture families to prove `13` frontend obligations.
4. Explicit list of deferred topics that belong to future backend-focused agenda.

## Candidate Decisions

### 1. Builtin Surfaces Are Frontend-Validated Before Any Backend Contract

Candidate direction:

- builtin names and metadata usage are validated at frontend scope per existing specs,
- and lowering emits deterministic outcomes or deterministic rejection.

### 2. Diagnostics Contract Is Mandatory For Frontend-Lowering Failures

Candidate direction:

- code/severity/primary attribution are fixed and test-backed for required rejection classes,
- enabling reliable regression protection.

### 3. Gate Evidence Is Small But Mandatory

Candidate direction:

- each required obligation has at least one positive and one negative fixture where applicable,
- with assertions for lexer/parser/AST/IRBackend/diagnostics.

### 4. Backend Topics Are Explicitly Deferred

Candidate direction:

- VM lowering, bytecode mapping, runtime, verifier, and loader remain out of this agenda,
- and are listed as deferred backlog topics.

## Questions To Resolve In The Room

1. Which builtin scenarios are mandatory in v1 fixture corpus?
2. Which lowering-related diagnostics are mandatory to freeze now?
3. What is the minimum evidence bar to declare `13` frontend slice closed?
4. Which open points are explicitly deferred without blocking frontend progress?

## Expected Outputs

1. a decision note on builtin frontend-lowering obligations,
2. a decision note on diagnostics/test evidence obligations,
3. and a closure checklist for `13` frontend scope.

## Explicit Deferrals

- VM lowering design,
- bytecode/PBX mapping,
- runtime execution contracts,
- verifier and loader internals.

## Inputs

- `docs/pbs/specs/6.1. Intrinsics and Builtin Types Specification.md`
- `docs/pbs/specs/12. Diagnostics Specification.md`
- `docs/pbs/specs/13. Lowering IRBackend Specification.md`
- `docs/general/specs/13. Conformance Test Specification.md`
- `docs/general/specs/19. Verification and Safety Checks Specification.md`
- `docs/pbs/agendas/13.3. IR and Lowering Workshop 3 - Callables, Services, Contracts, and Callbacks.md`