# PR-014 - PBS Switch Wildcard Rule Enforcement

## Briefing

Switch semantics do not currently enforce all wildcard constraints. This PR enforces: at most one wildcard arm, and rejection of mixed `default` and `_` in the same switch.

## Motivation

Wildcard ambiguity weakens deterministic static semantics and conformance.

## Target

- Switch expression semantic validation in flow analyzer.
- Required static diagnostics for wildcard rule violations.

## Scope

- Enforce wildcard-count and wildcard-mixing rules.
- Preserve existing selector/type compatibility checks.

## Method

- Track wildcard token form used per switch arm.
- Emit diagnostics for duplicate wildcard and mixed wildcard spellings.
- Keep existing exhaustiveness behavior.

## Acceptance Criteria

- Two wildcard arms are rejected deterministically.
- `default` + `_` in one switch is rejected deterministically.
- Existing duplicate-pattern logic for non-wildcards remains unchanged.

## Tests

- Add tests for duplicate wildcard arms.
- Add tests for mixed `default`/`_` arms.
- Run semantic control-flow suite.

## Non-Goals

- New switch syntax.
- Runtime switch lowering changes.