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Don't obtain apparent type of contextual types being optional type variables #61635

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Don't obtain apparent type of contextual types being optional type variables #61635

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fixes #61633

@Copilot Copilot AI review requested due to automatic review settings April 30, 2025 21:50
@github-project-automation github-project-automation bot moved this to Not started in PR Backlog Apr 30, 2025
@typescript-bot typescript-bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Apr 30, 2025
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Pull Request Overview

This PR addresses #61633 by fixing an incorrect treatment of contextual types that are optional type variables. The changes include updating the type checker logic to use a non‑nullable version of the instantiated type when verifying type variable status, and adding corresponding tests.

Reviewed Changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 1 comment.

File Description
tests/cases/compiler/inferenceFromGenericClass1.ts Adds tests to verify proper inference when using generic classes with optional type variables.
tests/baselines/reference/inferenceFromGenericClass1.js Updates baseline output corresponding to the new test behavior.
src/compiler/checker.ts Adjusts the type checking logic to use getNonNullableType and maybeTypeOfKind in the constraint check.
Files not reviewed (2)
  • tests/baselines/reference/inferenceFromGenericClass1.symbols: Language not supported
  • tests/baselines/reference/inferenceFromGenericClass1.types: Language not supported

src/compiler/checker.ts
@@ -32412,7 +32412,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
getContextualTypeForObjectLiteralMethod(node, contextFlags) :
getContextualType(node, contextFlags);
const instantiatedType = instantiateContextualType(contextualType, node, contextFlags);
if (instantiatedType && !(contextFlags && contextFlags & ContextFlags.NoConstraints && instantiatedType.flags & TypeFlags.TypeVariable)) {
if (instantiatedType && !(contextFlags && contextFlags & ContextFlags.NoConstraints && maybeTypeOfKind(getNonNullableType(instantiatedType), TypeFlags.TypeVariable))) {
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This change replaces the direct check on instantiatedType.flags with a call to maybeTypeOfKind on a non-nullable version of instantiatedType. Please double-check that using getNonNullableType ensures that optional type variables are not incorrectly inferred as type variables.

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  1. Without this instantiateTypeWithSingleGenericCallSignature computed contextualType as AnyConstructor | undefined
  2. then eliminated | undefined with getNonNullableType
  3. and obtained a generic contextualSignature from that
  4. and returned anyFunctionType because this happened with CheckMode.SkipGenericFunctions
  5. given anyFunctionType is non-inferrable, getInferredType picked up the default type argument (undefined)
  6. and getSignatureApplicabilityError returned with diagnostics because it was called with a check candidate that was a product of signature instantiation with currently inferred type arguments

@Andarist Andarist force-pushed the fix/apparent-type-of-contextual-optional-type-variable branch from f29693d to 529fc9e Compare April 30, 2025 21:51
@Andarist Andarist mentioned this pull request Apr 30, 2025
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Broken inference for generic classes passed to a generic function taking an optional class
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