Port TS PR 61668 - Fix type variable leaks and cache inconsistencies

[jakebailey]

Ports microsoft/TypeScript#61668.

I am not 100% sure this is correct. Might need @ahejlsberg to attempt it.

[Copilot]

Pull Request Overview

This PR ports TypeScript PR #61668, which fixes type variable leaks and cache inconsistencies in generic type inference. The changes focus on improving how type parameters are handled in signature instantiation to prevent incorrect type variable leakage between different inference contexts.

Key Changes:

• Simplified single signature type handling by removing the separate SingleSignatureType struct and treating these as anonymous types
• Enhanced outer return type mapping to prevent circular inferences and type parameter leaks
• Improved type mapper creation for better isolation of type parameters across inference contexts

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

:

File	Description
internal/checker/types.go	Removed SingleSignatureType struct and simplified type handling
internal/checker/checker.go	Modified signature instantiation logic and added outer return mapper functionality
internal/checker/inference.go	Added createOuterReturnMapper function for better type parameter isolation
internal/checker/nodebuilderimpl.go	Updated function call to remove outer type parameters parameter
internal/checker/jsx.go	Updated JSX signature creation calls to match new API
internal/testrunner/compiler_runner.go	Removed test skip for now-working inference test
testdata/baselines/reference/submodule/compiler/*.types	Updated type inference test baselines showing improved behavior

[ahejlsberg]

Approved with the suggested changes.