Normalize distributed type parameters in conditional type relationships - #64307
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Mateusz Burzyński (Andarist) wants to merge 3 commits into
Open
Mateusz Burzyński (Andarist) wants to merge 3 commits into
Mateusz Burzyński (Andarist) wants to merge 3 commits into
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Mateusz Burzyński (Andarist)
September 17, 2026 10:22
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🟢 Approval recommended
The focused normalization fix is consistent with the mapper behavior and has bidirectional regression coverage.
Pull request overview
Normalizes both sides of conditional true-branch relationships, restoring assignability for equivalent distributive conditional types.
Changes:
- Instantiates the target true type with the inference mapper.
- Adds a bidirectional regression test and generated baselines.
File summaries
| File | Description |
|---|---|
tsc/internal/checker/relater.go |
Normalizes the target conditional branch. |
tsc/testdata/tests/cases/conformance/types/conditional/conditionalTypes1.ts |
Adds regression coverage. |
tsc/testdata/baselines/reference/conformance/conditionalTypes1.types |
Updates type baseline. |
tsc/testdata/baselines/reference/conformance/conditionalTypes1.symbols |
Updates symbol baseline. |
tsc/testdata/baselines/reference/conformance/conditionalTypes1.js |
Updates emit and declaration baseline. |
tsc/testdata/baselines/reference/conformance/conditionalTypes1.errors.txt |
Confirms no new diagnostics. |
Review details
- Files reviewed: 6/6 changed files
- Comments generated: 0
- Review effort level: Balanced
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…non-distributed form Types declared inside a distributive conditional type reference the distributed form of the check type parameter. Instantiation maps those references back to the non-distributed form, but only for the parts of the type it actually rebuilds. Anonymous types and conditional types seed their instantiation cache with an identity entry that returns the declared type unchanged, so an inline object literal, function type, or nested conditional type kept the distributed type parameter after instantiation. Skip seeding that identity entry for types declared in a distributive context, and normalize both sides of a conditional-to-conditional relation (extends, true, and false types) with the non-distributing mapper so the comparison doesn't depend on whether either side has been instantiated.
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fixes a regression from #64237 , cc Anders Hejlsberg (@ahejlsberg)
The target type here needs to be instantiated too - so the
Tcan be mapped back to the non-distributed type parameter. The source type already does that mapping so the distributed type param on that side becomes non-distributed. But that creates an assymetry without mapping the target side too.