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Add a regression test for narrowing NoInfer union constituents of non-generic aliases - #64969

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Mateusz Burzyński (Andarist) wants to merge 1 commit into
microsoft:mainfrom
Andarist:andarist/narrowing-noinfer-union-constraint-test
Open

Mateusz Burzyński (Andarist) wants to merge 1 commit into
microsoft:mainfrom
Andarist:andarist/narrowing-noinfer-union-constraint-test

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@Andarist

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I found this missing~ coverage when playing around with: #64958 . I think the gains from that experiment are too little to justify pulling this in but the tests coverage is nice

Copilot AI balanced review requested due to automatic review settings October 26, 2026 12:24
@github-project-automation github-project-automation Bot moved this to Not started in PR Backlog Oct 26, 2026
@typescript-automation typescript-automation Bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Oct 26, 2026
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This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

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🟢 Approval recommended

The focused test-only additions have consistent baselines and no identified blocking issues.

No open issues 🎉

What changed in this PR

Adds compiler regression coverage for narrowing NoInfer union constituents in non-generic aliases, without changing compiler behavior.

Changes:

  • Tests discriminant narrowing directly and after accessing an alias through Box<string>.
  • Adds type and symbol baselines recording the expected results.
File Description
tsc/​testdata/​tests/​cases/​compiler/​narrowingNoInfer2.ts Adds both narrowing scenarios under strict checking.
tsc/​testdata/​baselines/​reference/​compiler/​narrowingNoInfer2.types Records expected narrowed types and property types.
tsc/​testdata/​baselines/​reference/​compiler/​narrowingNoInfer2.symbols Records expected symbol bindings.

Review effort: Balanced

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