# PR-015 - PBS this/Self Context Validation

## Briefing

The frontend accepts `this` and `Self` outside allowed contexts without required diagnostics. This PR adds explicit static checks for invalid usage.

## Motivation

Context-free acceptance of `this`/`Self` is non-conformant and creates type ambiguity.

## Target

- `this` expression analysis.
- `Self` type-surface analysis in declarations.
- Error catalog and deterministic diagnostics.

## Scope

- Reject `this` outside struct method, service method, and ctor bodies.
- Reject `Self` outside struct/service method declarations and ctors.

## Method

- Add callable-context metadata into flow/type validation.
- Emit dedicated diagnostics for invalid `this` and invalid `Self` usage.
- Keep valid owner contexts unchanged.

## Acceptance Criteria

- Invalid `this` usage emits deterministic static diagnostic.
- Invalid `Self` usage emits deterministic static diagnostic.
- Valid struct/service/ctor contexts remain accepted.

## Tests

- Add positive tests for valid `this`/`Self` contexts.
- Add negative tests for top-level functions and unrelated declarations.
- Run declaration + flow semantic suites.

## Non-Goals

- Introducing new receiver features.
- Altering `Self` syntax.