Contextually type array literal spread operands

[Sahil Singh Kushwah (WhitefistEmperor)]

Fixes #45600

Summary

Array literals used as spread operands were checked without the contextual type of the containing array or call. This caused literal properties to widen prematurely—for example, { field: "a" } became { field: string }—and produced an incorrect assignability error.

This change propagates the relevant array or tuple context into spread operands.

Approach

• Derive the contextual type for an array spread from the portion of the containing array or tuple that it occupies.
• Use the remaining parameter types to contextually type literal spread operands when a call has one non-generic candidate signature.
• Support parenthesized, conditional, ||, and ?? expression trees containing array literals.
• Preserve existing behavior for ordinary arrays and custom iterators by restricting signature-aware handling to contextually typable literal expression trees.

This also improves tuple precision in an existing Promise.all case from [number, ...void[]] to [number, void] and resolves an existing literal-freshness case marked Desired: OK.

Tests

Added regression coverage for:

• direct nested array spreads
• conditional spreads
• tuple slices
• tuples constructed from a single spread
• multiple spreads
• rest-parameter calls
• fixed-parameter calls
• invalid elements that must continue to produce an error

The new regression produces no diagnostics for valid cases and exactly one TS2322 diagnostic for the intentional invalid control.

Validation performed:

• focused regression test
• 26 related compiler regression tests
• complete compiler baseline suite
• native-preview tests: 12/12 passed
• compiler and tooling lint: 0 issues
• generation and formatting checks

npx hereby validate completed all compiler, native-preview, lint, generation, and formatting stages successfully. The only local failures were the Windows fswatch symlink tests because the current account does not have permission to create symbolic links.

AI assistance disclosure

I used OpenAI Codex to assist with investigating, implementing, and testing this change. I reviewed the resulting implementation and tests, understand the design and tradeoffs, and will personally respond to review feedback.

[Copilot]

Copilot review overview

🟡 Changes recommended

Multiple call spreads can receive parameter context from the wrong position, and two new expression paths lack regression coverage.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Use effective argument position for tuple spread contextual typing · New
• Add regression tests for spread calls with || and ?? · New

What changed in this PR

Propagates contextual types into array spread operands to preserve literal and tuple precision.

Changes:

• Adds spread-context derivation for arrays, tuples, and calls.
• Supports nested conditional/logical literal expressions.
• Adds regression tests and updates affected baselines.

File	Description
tsc/​internal/​checker/​checker.go	Implements contextual spread typing.
tsc/​internal/​checker/​utilities.go	Updates call-argument API usage.
tsc/​testdata/​tests/​cases/​compiler/​contextualTypingArraySpreadElements.ts	Adds regression cases.
tsc/​testdata/​baselines/​reference/​compiler/​contextualTypingArraySpreadElements.types	Records inferred types.
tsc/​testdata/​baselines/​reference/​compiler/​contextualTypingArraySpreadElements.symbols	Records symbols.
tsc/​testdata/​baselines/​reference/​compiler/​contextualTypingArraySpreadElements.js	Records emitted JavaScript.
tsc/​testdata/​baselines/​reference/​compiler/​contextualTypingArraySpreadElements.errors.txt	Records expected diagnostic.
tsc/​testdata/​baselines/​reference/​compiler/​literalFreshnessPropagationOnNarrowing.types	Updates literal freshness types.
tsc/​testdata/​baselines/​reference/​compiler/​literalFreshnessPropagationOnNarrowing.errors.txt	Removes resolved diagnostic.
tsc/​testdata/​baselines/​reference/​compiler/​awaitedType.types	Updates Promise.all tuple precision.

[Sahil Singh Kushwah (WhitefistEmperor)]

@microsoft-github-policy-service agree