# PR-23 — Eliminate Invalid Call Fallbacks

### Goal

Prevent invalid bytecode generation.

### Required Changes

* Remove **all** fallbacks to `FunctionId(0)` or equivalent
* On unresolved symbols during lowering:

  * Emit canonical diagnostic (`E_RESOLVE_UNDEFINED` or `E_LOWER_UNSUPPORTED`)
  * Abort lowering

### Tests

* PBS program calling missing function → compile error
* No Core IR or VM IR emitted

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# PR-24 — Validate Contract Calls in Frontend (Arity + Types)

### Goal

Move contract validation to compile time.

### Required Changes

* During PBS type checking:

  * Validate argument count against contract signature
  * Validate argument types

* Lower only validated calls to `HostCall`

### Tests

* Wrong arity → `E_TYPE_MISMATCH`
* Correct call lowers to Core IR `HostCall`

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# PR-25 — Core IR Invariants Test Suite

### Goal

Lock in correct semantics before touching the VM.

### Required Invariants

* Every `Begin*` has a matching `End*`
* Gate passed to `Begin*` is available at `End*`
* No storage writes without `BeginMutate`
* No silent fallbacks

### Tests

* Property-style tests or golden IR assertions

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## STOP POINT

After PR-25:

* Core IR correctly represents PBS HIP semantics
* Lowering is deterministic and safe
* VM is still unchanged

**Only after this point may VM PRs begin.**

Any VM work before this is a hard rejection.

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## Instruction to Junie

If any rule in this document is unclear:

* Stop
* Add a failing test
* Document the ambiguity

Do not invent behavior.

This document is binding.