Extra errors when using invalid things with `model is`

[timotheeguerin]

checkModelIs doesn't call pendingResolutions.finish on early-return error paths

Summary

In packages/compiler/src/core/checker.ts, the checkModelIs function calls pendingResolutions.start(modelSymId, ResolutionKind.BaseType) at the beginning but several early-return error paths skip the corresponding pendingResolutions.finish(...) call. This leaves the resolution in a "started" state, causing spurious circular-base-type diagnostics when another model later references the affected model via is or extends.

Repro

model A is { x: int32 };
model B is A;

Expected: Only one diagnostic — is-model error on A (model expression not allowed with is).

Actual: Two diagnostics:

• is-model: Model is cannot specify a model expression.
• circular-base-type: Type 'A' recursively references itself as a base type. ← spurious

Similarly:

model A is "hello";
model B is A;

Produces a spurious circular-base-type for A.

Root Cause

In checkModelIs (~line 6397), pendingResolutions.start is called unconditionally, but three early-return branches don't call finish:

1. Model expression (line ~6415): return undefined after reporting is-model with modelExpression message
2. Circular reference detected (line ~6430): return undefined after reporting circular-base-type
3. Other invalid expression (line ~6435): return undefined after reporting is-model

The finish call only exists after the if/else block (line ~6438), which is unreachable on these paths.

Fix

Add pendingResolutions.finish(modelSymId, ResolutionKind.BaseType) before each early return undefined in checkModelIs.

Test coverage

Regression tests have been added in packages/compiler/test/checker/model.test.ts under "pending resolution is finished on error paths" (currently marked with it.fails). Once fixed, flip them to regular it(...).

[github-actions]

🎯 Issue Triage

When model is references an invalid type (e.g., a model expression or a non-model type), the compiler incorrectly leaves a pending resolution open, causing spurious circular-base-type errors on other models that use is or extends on the affected model.

Category: Bug
Area: compiler:core

🔗 Reproduction:

📋 Reproduction code

// Case 1: model is model expression
model A is { x: int32 };
model B is A;  // spurious circular-base-type error on B

// Case 2: model is non-model type
model A is "hello";
model B is A;  // spurious circular-base-type error on B

// Case 3: circular reference + extends
model A is B;
model B is A;
model C extends A {}  // spurious second circular-base-type error on C

🔍 Analysis

• Root cause: checkModelIs does not call pendingResolutions.finish on all early-return (error) paths. When it exits early due to an invalid is target, the pending resolution remains open. Subsequent type checks for models that reference this model via is or extends then incorrectly detect it as a circular base type.
• Fix: Ensure pendingResolutions.finish is called on every exit path of checkModelIs, including error paths.
• The issue includes failing test cases that describe the expected behavior.

📝 Suggested next steps

• Fix checkModelIs in the compiler to call pendingResolutions.finish on all early-return paths
• Ensure the failing tests in the issue body pass after the fix

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