[Oleksandr Tarasiuk (a-tarasyuk)]

Fixes #64974

[Copilot]

🟢 Approval recommended

Static review identified no blocking issues in the focused fix and regression baselines; automated validation was not run.

No open issues 🎉

What changed in this PR

Fixes #64974 by preserving contextual types for destructured parameter defaults, preventing incorrect TS7031 errors.

Changes:

• Skip object-literal padding when the root parameter has a contextual type.
• Add regression cases for optional parameters, nested destructuring, arrays, index signatures, and generic callbacks.
• Update type, symbol, and diagnostic baselines.

File	Description
tsc/​testdata/​tests/​cases/​compiler/​objectBindingPatternDefaultMissingElements.ts	Adds contextual-typing regression cases.
tsc/​testdata/​baselines/​reference/​compiler/​objectBindingPatternDefaultMissingElements.types	Records expected inferred types.
tsc/​testdata/​baselines/​reference/​compiler/​objectBindingPatternDefaultMissingElements.symbols	Records symbols for the added cases.
tsc/​testdata/​baselines/​reference/​compiler/​objectBindingPatternDefaultMissingElements.errors.txt	Records expected diagnostics without spurious implicit-any errors.
tsc/​internal/​checker/​checker.go	Guards object-literal padding to preserve contextual typing.

_{Review effort: Balanced}

[timurzvural]

Opened #65016 as an alternative that also covers #65011.

Same contextual-type guard for object padding; additionally guards array padding and adds JSDoc @returns / cast / annotated .ts cases from #65011 on top of the #64974 scenarios.

[Neeraj Dalal (nrjdalal)]

One more shape for the test file, from a codebase that hit this on 7.1: a pattern where some binding elements have their own defaults. Only the elements without a default are flagged; the ones with a default keep their type.

// tsconfig: { "compilerOptions": { "strict": true, "noEmit": true, "target": "esnext", "module": "esnext", "types": [] } }
type Run = (argv: string[], options?: { cwd?: string; timeout?: number }) => void

export const run: Run = (argv, { cwd, timeout = 1 } = {}) => {
  const n: number = timeout
  console.log(argv, cwd, n)
}

Version	Result
7.0.2	no errors
7.1.0-beta	(4,34): error TS7031: Binding element 'cwd' implicitly has an 'any' type.
7.1.0-dev.20261029.1	same

f1 here covers the plain ({ a, b } = {}) form. A case like ({ a, b = 1 } = {}) would pin the mixed form too. With this PR's guard, cwd should read string | undefined and timeout should read number, as on 7.0.2.

[timurzvural]

Good catch — that mixed form is the same regression.

On 7.1, padding invents any for missing properties on the = {} default and overrides the contextual parameter type, so cwd becomes implicit any (TS7031) while timeout = 1 keeps a usable type from its own initializer. With the contextual-type guard (skip padObjectLiteralType when the root parameter already has a contextual type), cwd should be string | undefined and timeout should be number, matching 7.0.2.

I opened #65016 as an alternative that uses the same object-padding guard (and also guards array padding). I will add an explicit ({ cwd, timeout = 1 } = {}) / ({ a, b = 1 } = {}) case there so the mixed shape is pinned in baselines, not only the plain ({ a, b } = {}) form from #64974.