Fix panic when inferring [...rest, ...T] from tuple shorter than fixed-arity constraint

[Copilot]

Go slices panic on negative indices, unlike JS Array.slice which clamps to 0. In the [...rest, ...T] inference branch, startIndex = endIndex - impliedArity goes negative when the source tuple is shorter than T's constraint arity.

function f<T extends [string]>(args: [...string[], ...T]) {}
f([]) // panic: slice bounds out of range [-1:]

• Added bounds check startIndex >= startLength before slicing source tuple type arguments in inferFromObjectTypes
• Added compiler test case reproducing the crash

[ahejlsberg]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved the merge conflict in internal/checker/inference.go — combined the bounds check (startIndex >= startLength) with the restType != nil guard from main. All tests, build, lint, and format pass. (ba1dd4d)

[Copilot]

Pull request overview

Fixes a Go panic in the [...rest, ...T] tuple inference branch when the source tuple is shorter than the fixed-arity constraint of T. The Go-native slice c.getTypeArguments(source)[startIndex:endIndex] panics on negative startIndex, while JavaScript's Array.slice would clamp; the fix guards the slice with startIndex >= startLength before performing inference, and adds a regression compiler test.

Changes:

• Add startIndex >= startLength bounds check around the [...rest, ...T] trailing slice/inference block in inferFromObjectTypes.
• Add inferRestTupleShortSource.ts compiler test plus accepted baselines (.types, .symbols, .js, .errors.txt).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/inference.go	Wraps trailing-slice creation and both inferFromTypes calls in a startIndex >= startLength guard to avoid negative slice bounds.
testdata/tests/cases/compiler/inferRestTupleShortSource.ts	New regression test reproducing the panic with f<T extends [string]>(args: [...string[], ...T]) called as f([]).
testdata/baselines/reference/compiler/inferRestTupleShortSource.types	Accepted types baseline showing f([]) resolves to void with no crash.
testdata/baselines/reference/compiler/inferRestTupleShortSource.symbols	Accepted symbols baseline for the new test.
testdata/baselines/reference/compiler/inferRestTupleShortSource.js	Accepted JS emit baseline for the new test.
testdata/baselines/reference/compiler/inferRestTupleShortSource.errors.txt	Accepted error baseline showing TS2345 with target [...string[], string] (T defaults to its constraint).