Control flow analysis for destructured rests #62350
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typescript-bot
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For Uncommitted Bug
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The TypeScript team hasn't accepted the linked issue #46680. If you can get it accepted, this PR will have a better chance of being reviewed. |
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The TypeScript team hasn't accepted the linked issue #46680. If you can get it accepted, this PR will have a better chance of being reviewed. |
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Pull Request Overview
This PR implements control flow analysis for destructured rest variables, addressing two specific TypeScript issues (#46680 and #53947). The changes enable the TypeScript compiler to properly narrow types in destructuring assignments that include rest elements, improving type inference when discriminated unions are involved.
Key Changes
- Enhanced control flow analysis to handle destructured rest variables by tracking narrowed types through rest patterns
- Modified the constraint substitution logic to be more selective, improving type narrowing for binding elements
- Extended support for both object and array destructuring with rest elements
Reviewed Changes
Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| tests/cases/conformance/controlFlow/dependentDestructuredRestVariables1.tsx | New test cases demonstrating control flow analysis for destructured rest variables in various scenarios |
| tests/baselines/reference/dependentDestructuredRestVariables1.symbols | Generated symbol baseline for the new test cases |
| tests/baselines/reference/arrayDestructuringInSwitch1.types | Updated type baseline showing improved type inference for array destructuring |
| tests/baselines/reference/arrayDestructuringInSwitch1.symbols | Updated symbol baseline with refined symbol references |
| src/compiler/checker.ts | Core implementation changes to enable control flow analysis for rest variables |
Comments suppressed due to low confidence (1)
src/compiler/checker.ts:1
- [nitpick] The long conditional expression on line 30851 reduces readability. Consider extracting the check mode determination into a variable:
const checkMode = shouldSubstituteConstraints(type, location) ? CheckMode.Normal : CheckMode.SkipConstraintsSubstitution;
import {
Andarist
commented
Aug 28, 2025
| const parent = declaration.parent.parent; | ||
| const rootDeclaration = getRootDeclaration(parent); | ||
| if (rootDeclaration.kind === SyntaxKind.VariableDeclaration && getCombinedNodeFlagsCached(rootDeclaration) & NodeFlags.Constant || rootDeclaration.kind === SyntaxKind.Parameter) { | ||
| const links = getNodeLinks(parent); | ||
| if (!(links.flags & NodeCheckFlags.InCheckIdentifier)) { | ||
| links.flags |= NodeCheckFlags.InCheckIdentifier; | ||
| const parentType = getTypeForBindingElementParent(parent, CheckMode.Normal); | ||
| const parentTypeConstraint = parentType && mapType(parentType, getBaseConstraintOrType); | ||
| const parentType = getTypeForBindingElementParent(parent, shouldSubstituteConstraints(type, location) ? CheckMode.Normal : CheckMode.SkipConstraintsSubstitution); |
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When obtaining the parent type we don't want to get its constraints substituted based on its contextual type or it being in a constraint position or something. This would lead to returning its constraint and then using that for getBindingElementTypeFromParentType even when the current location isn't at the constraint position.
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Huh, I'll have to investigate what's happening with the self-check |
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wowwww ❤️ |
Andarist
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Aug 30, 2025
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| === Performance Stats === | ||
| Assignability cache: 2,500 | ||
| Type Count: 10,000 | ||
| Instantiation count: 100,000 | ||
| Symbol count: 50,000 |
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fwiw, those numbers are the same on main
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@typescript-bot test it |
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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
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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 #46680
fixes #53947