[jasleenkaur-qed42]

Summary

Fixes a 7.1 regression where assignments between instantiations of mutually recursive type aliases were accepted even when their type arguments are incompatible.

Root Cause

During variance measurement, a comparison that reaches a generic type whose variances are still being computed returns TernaryUnknown, i.e. succeeds. When every occurrence of a type parameter is reached through such a type, the sub/super/unrelated marker probes all succeed and the parameter is recorded as Independent, so typeArgumentsRelatedTo ignores it.

microsoft/typescript-go#4820 made cycle handling deterministic by restarting variance computation at the earliest declared symbol in the cycle. In the repro that is Params, so Handler is always measured while Params is in progress, and both of Handler's type parameters end up Independent. Before #4820 the same thing happened only when Params was compared first, which is why that order misses the error in every version.

Changes

• Track when a variance probe relied on an in-progress (circular) variance lookup.
• In that case, record the parameter as Bivariant | Unmeasurable instead of Independent. Identical arguments are still related directly; otherwise we fall back to a structural comparison, so parameters that are truly unwitnessed are still accepted.
• New tests circularVariance3.ts and circularVariance4.ts cover both comparison orders, including an unwitnessed parameter that must remain assignable.
• varianceMeasurement.errors.txt changes only in error elaboration for the Assignability check not enough for class that is mutually recursive with an interface? #39947 repro; the same error is still reported.

Testing

• go test ./internal/testrunner/ and go test ./internal/checker/ pass.
• go test ./... passes apart from internal/astnav (mapCode.ts), which fails the same way without this change and doesn't depend on the checker.

Manual Verification

1. Build from this branch and run tsgo -p . on the repro from the issue.
2. Confirm TS2322 for Handler<number, string> and Handler<boolean, { x: 1 }>.
3. Compare a Params instantiation before the Handler assignment and confirm both errors are still reported.
4. Confirm Handler<string, number> (only the unwitnessed parameter differs) is still accepted.

Related Issue

Closes #64930

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[Copilot]

🔵 Needs a closer look

Core recursive assignability changes warrant human review with confirmed full validation, which was not independently run here.

No open issues 🎉

What changed in this PR

Fixes the regression in #64930 by preventing circular variance lookups from marking type parameters as independent, which could incorrectly allow assignments between incompatible recursive types.

Changes:

• Track circular variance lookups and use structural comparison when independence cannot be established.
• Add regression tests for both comparison orders while preserving assignability for an unwitnessed parameter.
• Add test baselines and update diagnostic elaboration for the existing #39947 case.

File	Description
tsc/​testdata/​tests/​cases/​compiler/​circularVariance4.ts	Tests assignments after comparing Params first.
tsc/​testdata/​tests/​cases/​compiler/​circularVariance3.ts	Tests assignments when comparing Handler first.
tsc/​testdata/​baselines/​reference/​compiler/​varianceMeasurement.errors.txt	Updates error elaboration without changing the error count.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance4.types	Records types for the Params-first case.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance4.symbols	Records symbols for the Params-first case.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance4.errors.txt	Records three expected assignment errors.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance3.types	Records types for the Handler-first case.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance3.symbols	Records symbols for the Handler-first case.
tsc/​testdata/​baselines/​reference/​compiler/​circularVariance3.errors.txt	Records two expected assignment errors.
tsc/​internal/​checker/​relater.go	Detects circular lookups and marks affected variance as unmeasurable.
tsc/​internal/​checker/​checker.go	Adds state for tracking circular variance lookups.

_{Review effort: Balanced}