Fix crash related to tuple rest arity#2928
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When inferring from tuple types with a mix of rest and variadic elements (e.g. [...rest, ...T] or [...T, ...rest]), getElementTypeOfSliceOfTupleType can return nil when the variadic element's implied arity consumes all source tuple elements. The nil return value was passed directly to inferFromTypes, causing a nil pointer dereference at inference.go:79. Added nil guards consistent with the existing pattern used elsewhere in the same function (line ~736). When getElementTypeOfSliceOfTupleType returns nil, we skip the inference call since there are no remaining source elements to infer from. This bug also exists in the TypeScript compiler (tsc crashes with TypeError on the same input), so this fix diverges from the TS source to prevent the crash in Go.
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
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Pull request overview
This PR fixes a crash during compilation when inferring from tuple types with rest and variadic elements. The crash occurred when a variadic element's constraint consumed all source tuple elements, causing getElementTypeOfSliceOfTupleType to return nil, leading to a nil pointer dereference.
Changes:
- Added nil guards at two locations in
inferFromObjectTypeswheregetElementTypeOfSliceOfTupleTypecould return nil - Added comprehensive compiler test case
inferFromTupleRestAndVariadic.tscovering both[...rest, ...T]and[...T, ...rest]patterns - Accepted baselines showing correct type inference behavior
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| testdata/tests/cases/compiler/inferFromTupleRestAndVariadic.ts | New compiler test with edge cases for tuple inference with rest and variadic elements |
| testdata/baselines/reference/compiler/inferFromTupleRestAndVariadic.types | Type inference baseline showing correct results for all test cases |
| testdata/baselines/reference/compiler/inferFromTupleRestAndVariadic.symbols | Symbol resolution baseline for the test cases |
| testdata/baselines/reference/compiler/inferFromTupleRestAndVariadic.js | JavaScript emit baseline for the test cases |
| internal/checker/inference.go | Added nil guards before calling inferFromTypes with result from getElementTypeOfSliceOfTupleType |
jakebailey
reviewed
Apr 16, 2026
Comment on lines
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| if restType := c.getElementTypeOfSliceOfTupleType(source, startLength+impliedArity, endLength, false, false); restType != nil { | ||
| c.inferFromTypes(n, restType, elementTypes[startLength+1]) | ||
| } |
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Strada had:
// Middle of target is [...T, ...rest] and source is tuple type
// if T is constrained by a fixed-size tuple we might be able to use its arity to infer T
const param = getInferenceInfoForType(elementTypes[startLength])?.typeParameter;
const constraint = param && getBaseConstraintOfType(param);
if (constraint && isTupleType(constraint) && !(constraint.target.combinedFlags & ElementFlags.Variable)) {
const impliedArity = constraint.target.fixedLength;
inferFromTypes(sliceTupleType(source, startLength, sourceArity - (startLength + impliedArity)), elementTypes[startLength]);
inferFromTypes(getElementTypeOfSliceOfTupleType(source, startLength + impliedArity, endLength)!, elementTypes[startLength + 1]);
}Why is this change correct? The old code didn't nil check. Is this also a crash in Strada?
(Not sure in what context this PR was created.)
RyanCavanaugh
changed the title
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|
Hoisting comment: This is a fix for microsoft/TypeScript#63199 |
jakebailey
approved these changes
May 15, 2026
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infer+ rest/variadic tuple elementsinferFromObjectTypeswhengetElementTypeOfSliceOfTupleTypereturns nilinternal/checker/inference.goinferFromTupleRestAndVariadic.tswith baselines🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.