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Skip combined-constraint check while measuring variances to avoid unbounded chase #64528

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Skip combined-constraint check while measuring variances to avoid unbounded chase #64528

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5b7db18
Skip combined-constraint check while measuring variances
balyo-alexis-delrieu Sep 29, 2026
f326b55
Add regression test for the effective-constraint chase during varianc…
balyo-alexis-delrieu Oct 5, 2026
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10 changes: 9 additions & 1 deletion tsc/internal/checker/relater.go
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Original file line number Diff line number Diff line change
Expand Up @@ -3239,7 +3239,15 @@ func (r *Relater) structuredTypeRelatedTo(source *Type, target *Type, reportErro
// needs to have its constraint hoisted into an intersection with said type parameter, this way
// the type param can be compared with itself in the target (with the influence of its constraint to match other parts)
// For example, if `T extends 1 | 2` and `U extends 2 | 3` and we compare `T & U` to `T & U & (1 | 2 | 3)`
if result == TernaryFalse && (source.flags&TypeFlagsIntersection != 0 || source.flags&TypeFlagsTypeParameter != 0 && target.flags&TypeFlagsUnion != 0) {
// We skip the combined-constraint check while measuring variances. Variance
// measurement compares marker instantiations, and there the hoisted constraint may
// itself be an intersection with instantiable constituents (e.g. deferred conditional
// types, as produced by heavily overloaded generic builder types), so a failed
// comparison re-enters this logic with a structurally new and ever larger source
// that the relation cache and the deeply-nested-type guards never recognize as
// recursive, and the chase grows without bound (see microsoft/TypeScript#64423).
// The check never contributes to the measured relation between the marker types.
if result == TernaryFalse && len(r.c.varianceStack) == 0 && (source.flags&TypeFlagsIntersection != 0 || source.flags&TypeFlagsTypeParameter != 0 && target.flags&TypeFlagsUnion != 0) {

@Amatewasu Alexis Delrieu (Amatewasu) Oct 5, 2026

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LLM-generated test added

Regression test checked in as bd456af6e3 (tests/cases/compiler/effectiveConstraintChaseDuringVarianceMeasurement.ts plus .types/.symbols baselines), in response to the review comment asking for a checked-in test for the variance-stack guard.

  1. Minimized from the real repro. It is hand-distilled from the three.js TSL node types that triggered TypeScript 7.0.2 silently runs out of memory while typechecking a project that TypeScript 6.0.3 checks successfully #64423. Only the constructs needed to drive the combined-constraint comparison in structuredTypeRelatedTo during variance measurement were kept — resolving uv().div(vec2()) measures the variances of the generic extension interfaces, whose hoisted constraints form deferred conditional intersections.

  2. It fails without the fix. On the pre-fix revision (673a5f17d7) this file enters the unbounded relation chase and the process dies with runtime: out of memory. Removing or weakening the variance-stack guard reintroduces this immediately, so CI catches it instead of the regression going unnoticed.

  3. It passes with the fix and pins the exact output. With the guard, the file checks to completion in under a second with no diagnostics, and the baselines record the precise inferred types (e.g. x : Node<"vec2">). This also guards the opposite failure mode: a future change that "fixes" a hang by making the checker give up and infer different types would fail the baseline comparison.

Reproducing the pre-fix failure locally:

git show 673a5f17d7:tsc/internal/checker/relater.go > tsc/internal/checker/relater.go
cd tsc && GOMAXPROCS=2 timeout 120 go test ./internal/testrunner \
  -run 'TestLocal/effectiveConstraintChaseDuringVarianceMeasurement'
# pre-fix: never terminates (chase grows until OOM) / fixed: "ok" in ~1s

@Amatewasu Alexis Delrieu (Amatewasu) Oct 7, 2026

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What do you think Jake Bailey (@jakebailey)? :)

var sourceTypes []*Type
if source.flags&TypeFlagsIntersection != 0 {
sourceTypes = source.Types()
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