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fix(checker): apply discriminant-based contextual typing for object literals (fixes #1727) #1734

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fix(checker): apply discriminant-based contextual typing for object literals (fixes #1727) #1734

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Summary

This PR fixes issue #1727 where tsgo failed to contextually type callback parameters in object literals
when a discriminant property (e.g., visit: 'Declaration') was present. Unlike tsc, tsgo previously
gave the enter(node) parameter an implicit any type.

Details

  • When the contextual type of an object literal is a union of object types with a discriminant property:
    • Narrow the union to the constituent type that matches the literal discriminant value.
    • Use the narrowed type when applying contextual typing to other members (e.g., function parameters).
  • Fallback: if the discriminant is missing or ambiguous, fall back to current behavior.

Tests

  • Added baseline tests:
    1. Matching discriminant (visit: 'Declaration') → enter(node) is properly typed, no TS7006 error.
    2. Absent/mismatching discriminant → falls back to existing behavior (parity with tsc).

Notes

@Copilot Copilot AI review requested due to automatic review settings September 20, 2025 13:07
@arash-mosavi
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@microsoft-github-policy-service agree

Copilot
Copilot AI reviewed Sep 20, 2025
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Pull Request Overview

This PR fixes discriminant-based contextual typing for object literals in union types. Previously, tsgo failed to properly type callback parameters when a discriminant property was present, unlike the reference TypeScript compiler.

  • Adds reverse inference from literal types to indexed access types for discriminated unions
  • Implements type narrowing based on discriminant property values in object literals
  • Falls back to existing behavior when discriminants are missing or ambiguous

arash-mosavi and others added 3 commits September 20, 2025 19:12
@arash-mosavi @Copilot
Update internal/checker/inference.go
86c8830 
Co-authored-by: Copilot <[email protected]>
@arash-mosavi
fix(checker): improve inference handling for indexed access types
f3c8dca
@arash-mosavi
Add test case and baselines for cssTreeTypeInference
94f61a6 
- Add cssTreeTypeInference.ts test case to verify discriminated union type inference
- Include baseline files showing expected type inference behavior
- Ensures node parameter is correctly inferred as Declaration type in contextual typing scenarios
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Copilot AI reviewed Sep 20, 2025
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Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Copilot
Copilot AI reviewed Sep 20, 2025
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Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

@arash-mosavi
fix(checker): use TypeFlagsTypeVariable for consistency with getInfer…
5a88d66 
…enceInfoForType

- Change from TypeFlagsTypeParameter to TypeFlagsTypeVariable to match the pattern used in getInferenceInfoForType function
- This ensures consistency in type flag checking across inference-related functions
- TypeFlagsTypeVariable includes both type parameters and indexed access types, which is more comprehensive
- Updated comment to reflect the more accurate terminology 'type variable' instead of 'type parameter'
@arash-mosavi
perf(checker): optimize blockedStringType check in union member loop
e500bd5 
- Move blockedStringType check to beginning of loop for early exit
- Avoid expensive getIndexedAccessType() and isTypeIdenticalTo() calls when union member is blocked
- Improves performance by skipping unnecessary computations for blocked string types
- No functional changes, pure optimization
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Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

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Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

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Copilot AI reviewed Sep 20, 2025
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Pull Request Overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

@arash-mosavi
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Hello,

This pull request addresses #1727 by adding discriminant-based contextual typing for
object literals (e.g., visit: 'Declaration' now correctly narrows the union before typing
enter(node)), aligning the behavior with tsc 5.8.

  • CLA signed
  • Baseline tests added (covering both matching and
    This pull request implements a new reverse inference algorithm in the type checker to improve type inference for indexed access types, particularly when a literal value is assigned to an indexed access type. This change addresses issues with discriminated union inference (such as in CSS tree-like ASTs), ensuring that type parameters are correctly inferred in scenarios like T['type'] = 'Declaration'. Comprehensive tests are also added to verify the fix.

Type inference improvements:

  • Added a new inference case in Checker.inferFromTypes to handle situations where the source is a literal type and the target is an indexed access type, enabling reverse inference from the literal to the type variable.
  • Implemented the inferFromLiteralToIndexedAccess method, which infers the correct union member for a type parameter by matching the literal value with the indexed property type of union members. This is especially useful for discriminated unions.

Testing and validation:

  • Added a new test case (cssTreeTypeInference.ts) that reproduces a real-world issue with CSS tree type inference, ensuring that the fix works as intended.
  • Included baseline files for symbols and types to verify correct inference and symbol resolution in the new test scenario. [1] [2]
  • Updated the JavaScript baseline to include the expected runtime output for the new test case. fallback scenarios)
  • Local CI passing (pending workflow approval here)

Could you please approve the workflows and review this change?

@jakebailey — for workflow approval/merge
@weswigham or @sheetalkamat — for a checker/inference review

Thank you!

@ahejlsberg
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@arash-mosavi Appreciate your PR, but unfortunately I don't think we want to merge it. See my explanation here: #1727 (comment).

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Successfully merging this pull request may close these issues.

tsc vs tsgo: inference mismatch of parameter in passed callback
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@arash-mosavi @ahejlsberg