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Propagate variance reliability #62604

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Merged
merged 5 commits into from
Oct 31, 2025
+141 −21
Merged

Propagate variance reliability #62604

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d864b95
Propagate variance reliability in typeArgumentsRelatedTo
ahejlsberg Oct 15, 2025
f5298d6
Accept new baselines
ahejlsberg Oct 15, 2025
0942f04
Only propagate VarianceFlags.Unreliable
ahejlsberg Oct 15, 2025
f2e654a
Accept new baselines
ahejlsberg Oct 15, 2025
be235f2
Add regression test
ahejlsberg Oct 15, 2025
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8 changes: 6 additions & 2 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -23178,11 +23178,15 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// in the process of computing variance information for recursive types and when
// comparing 'this' type arguments.
const varianceFlags = i < variances.length ? variances[i] : VarianceFlags.Covariant;
const s = sources[i];
const t = targets[i];
// Propagate variance reliability flags
if (varianceFlags & (VarianceFlags.Unmeasurable | VarianceFlags.Unreliable)) {
instantiateType(s, varianceFlags & VarianceFlags.Unmeasurable ? reportUnmeasurableMapper : reportUnreliableMapper);
}
const variance = varianceFlags & VarianceFlags.VarianceMask;
// We ignore arguments for independent type parameters (because they're never witnessed).
if (variance !== VarianceFlags.Independent) {
const s = sources[i];
const t = targets[i];
let related = Ternary.True;
if (varianceFlags & VarianceFlags.Unmeasurable) {
// Even an `Unmeasurable` variance works out without a structural check if the source and target are _identical_.
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3 changes: 3 additions & 0 deletions tests/baselines/reference/circularlySimplifyingConditionalTypesNoCrash.types
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@@ -1,5 +1,8 @@
//// [tests/cases/compiler/circularlySimplifyingConditionalTypesNoCrash.ts] ////

=== Performance Stats ===
Instantiation count: 1,000

=== circularlySimplifyingConditionalTypesNoCrash.ts ===
type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
>Omit : Omit<T, K>
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2 changes: 1 addition & 1 deletion tests/baselines/reference/deeplyNestedMappedTypes.types
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Expand Up @@ -2,7 +2,7 @@

=== Performance Stats ===
Type Count: 1,000
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Copilot AI Oct 15, 2025

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Instantiation count increased from 5,000 to 25,000; please confirm this regression is intentional or add justification/mitigation (e.g. a note in the PR or follow-up optimization plan) since this may impact compile performance.

Suggested change
Type Count: 1,000
Type Count: 1,000
// NOTE: High instantiation count is intentional for this test case.
// This test is designed to stress-test recursive mapped types (see #55535).

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Instantiation count: 5,000
Instantiation count: 25,000

=== deeplyNestedMappedTypes.ts ===
// Simplified repro from #55535
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