Defer checking subexpressions of binary expressions with known boolean result type #60865
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src/compiler/checker.ts
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| // Those boolean arguments wouldn't even have to be viable sources for type arguments being inferred. | ||
| // | ||
| // For those reasons, we defer obtaining the operand types here and checking the related errors. | ||
| checkNodeDeferred(node); |
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This is really the core of the fix. The rest of the PR only moves code around (for the most part)
src/compiler/checker.ts
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| state.skip = true; | ||
| setLastResult(state, booleanType); | ||
| break; | ||
| case SyntaxKind.QuestionQuestionToken: |
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Out of curiosity (this may be a dumb question): Since SyntaxKind.EqualsToken (which I assume refers to assignment) is handled above and SyntaxKind.QuestionQuestionToken is handled here, should nullish coalescing assignment (SyntaxKind.QuestionQuestionEqualsToken) also be handled here?
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It's a valid question. I think no adjustments have to be made here though. Only = is permitted when the left side is a destructuring pattern. Other kinds of assignment~ operators in such scenarios result in a syntax error:
let a: number | undefined;
({ a } ??= { a: 10 }); // Uncaught SyntaxError: Invalid left-hand side in assignment
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Pull Request Overview
This PR refactors the TypeScript compiler's handling of binary expressions with known boolean result types by removing a recently added mechanism in favor of the older checkDeferredNode approach. The change aims to reduce redundant type checking errors, particularly in control flow analysis scenarios where binary expressions like comparisons have predetermined boolean types.
- Removes the
CheckMode.TypeOnlyflag mechanism introduced in a previous PR - Switches to using
checkDeferredNodefor binary expressions with boolean results - Adds specific handling for comparison and equality operators to defer type checking until later
Reviewed Changes
Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
src/compiler/types.ts |
Adds NullishCoalesceExpression interface for type safety |
src/compiler/checker.ts |
Major refactoring of binary expression checking, removes CheckMode.TypeOnly, adds deferred checking for boolean operators |
| Test files | Adds new test cases for control flow inference and updates existing baselines |
Comments suppressed due to low confidence (3)
fixes #60130
fixes #62279
This PR removes the mechanism introduced in #54380 in favor of using the older
checkDeferredNodemechanism that allows the compiler to avoid an extra set of redundant errors.