Discard types that reduce to never before discriminating by discriminable items
#62275
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Pull Request Overview
This PR fixes an issue where TypeScript's type discrimination was not properly handling union types that reduce to never. The fix ensures that types reducing to never are discarded before performing discrimination by discriminable items.
- Adds a compiler test case demonstrating the fix for contextual typing by discriminable unions
- Updates the type checker to filter out types that reduce to
neverduring discrimination - Includes baseline test files showing the expected type behavior
Reviewed Changes
Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| tests/cases/compiler/contextuallyTypedByDiscriminableUnion2.ts | New test case demonstrating the fix for discriminable union typing with never-reducing types |
| tests/baselines/reference/contextuallyTypedByDiscriminableUnion2.types | Expected type output baseline for the new test case |
| tests/baselines/reference/contextuallyTypedByDiscriminableUnion2.symbols | Expected symbol output baseline for the new test case |
| src/compiler/checker.ts | Core fix that excludes never-reducing types during discrimination |
| interface EnableA { | ||
| readonly enableA: true; | ||
| // this introduces a conflicting property with some of the other members of MyComponentProps | ||
| // making relevant final union members reduced nevers |
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The comment should be more specific about which members have conflicting properties and how this leads to never types. Consider explaining the specific conflict between EnableA requiring enableB: true and DisableB requiring enableB: false.
Suggested change
| // making relevant final union members reduced nevers | |
| // This introduces a conflicting property: EnableA requires enableB: true, | |
| // but DisableB requires enableB: false. As a result, any union member combining | |
| // EnableA and DisableB is reduced to never, since enableB cannot be both true and false. |
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| interface EnableA { | ||
| readonly enableA: true; | ||
| // this introduces a conflicting property with some of the other members of MyComponentProps | ||
| // making relevant final union members reduced nevers |
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This comment contains unclear language. 'reduced nevers' should be 'reduce to never' for grammatical correctness and clarity.
Suggested change
| // making relevant final union members reduced nevers | |
| // making relevant final union members reduce to never |
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@typescript-bot test it |
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For clarity, this is an effect of #61828 |
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Hey @jakebailey, I've packed this into an installable tgz. You can install it for testing by referencing it in your and then running There is also a playground for this build and an npm module you can use via |
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Hey @jakebailey, the results of running the DT tests are ready. Everything looks the same! |
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@jakebailey Here are the results of running the user tests with tsc comparing There were infrastructure failures potentially unrelated to your change:
Otherwise... Everything looks good! |
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@jakebailey Here they are:
tscComparison Report - baseline..pr
System info unknown
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Developer Information: |
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@jakebailey Here are the results of running the top 400 repos with tsc comparing Everything looks good! |
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fixes #62256