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

[Andarist]

fixes #61633

[Copilot]

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

[Copilot]

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.

[Andarist]

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

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

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