Narrow generic conditional and indexed access return types when checking return statements

[Gabriela Araujo Britto (gabritto)]

Port of #56941 + #61359.

Pending: inferring conditional return types.

Fixes #33912.
Fixes #33014.

[Copilot]

🟡 Changes recommended

Type-reference wrappers and one indexed-access branch can currently produce unsound accepted returns.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Ports conditional and indexed-access return-type narrowing into the native TypeScript checker.

Changes:

• Adds control-flow-aware return checking and narrowing constraints.
• Records branch flow nodes for conditional expressions.
• Adds comprehensive compiler and Fourslash regression coverage.

File summaries

File	Description
tools/scripts/tsc/ast.json	Adds conditional branch flow fields.
tsc/internal/ast/ast_generated.go	Regenerates conditional-expression AST.
tsc/internal/binder/binder.go	Captures return-position branch flow.
tsc/internal/checker/checker.go	Implements return-type narrowing.
tsc/internal/checker/types.go	Adds narrowing metadata and links.
tsc/internal/fourslash/tests/returnTypeNarrowingAfterCachingTypes_test.go	Tests cached-type behavior.
tsc/testdata/tests/cases/compiler/dependentReturnType1.ts	Core narrowing scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType2.ts	JavaScript/JSDoc scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType3.ts	Real-world TypeScript scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType4.ts	Optional-parameter scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType5.ts	Indexed-access scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType6.ts	Narrowing eligibility scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType8.ts	Expression-bodied arrow scenario.
tsc/testdata/tests/cases/compiler/dependentReturnType9.ts	Switch exhaustiveness scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType10.ts	Primitive/object union scenario.
tsc/testdata/tests/cases/compiler/dependentReturnType11.ts	Embedded type-parameter scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType12.ts	Non-strict optionality scenarios.
tsc/testdata/tests/cases/compiler/dependentReturnType13.ts	Non-primitive restrictions.
tsc/testdata/baselines/reference/compiler/dependentReturnType1.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType1.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType1.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType2.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType2.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType2.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType3.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType3.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType3.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType4.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType4.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType4.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType5.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType5.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType5.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType6.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType6.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType6.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType8.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType8.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType9.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType9.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType9.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType10.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType10.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType11.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType11.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType11.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType12.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType12.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType12.types	Expected types.
tsc/testdata/baselines/reference/compiler/dependentReturnType13.errors.txt	Expected diagnostics.
tsc/testdata/baselines/reference/compiler/dependentReturnType13.symbols	Expected symbols.
tsc/testdata/baselines/reference/compiler/dependentReturnType13.types	Expected types.

Review details

Files not reviewed (1)

• tsc/internal/ast/ast_generated.go: Generated file

• Files reviewed: 51/52 changed files
• Comments generated: 2
• Review effort level: Balanced