Narrow destructured discriminated unions when the discriminant has a default

[DukeDeSouth]

A default on the discriminant of a destructured union disables narrowing for the sibling bindings - { isText = false, children }: Props leaves children as string | number in both branches. It now still narrows when the default can never fire (the property is required and non-undefined across the union). Optional or possibly-undefined discriminants, and siblings with their own default, stay unchanged. Added a conformance test covering both.

Fixes #50139

[typescript-automation]

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[typescript-automation]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a regression test and compiler fix to ensure discriminant narrowing still works when a required discriminant is destructured with a default initializer, while preserving existing soundness boundaries for optional/possibly-undefined discriminants.

Changes:

• Added a new conformance test covering narrowing behavior with required discriminants that have destructuring defaults (plus negative/soundness cases).
• Updated checker.ts to allow dependent narrowing through binding elements with defaults only when the default can never apply (property always present and non-undefined across the declared union).
• Added corresponding reference baselines (.types, .symbols, .errors.txt) for the new test.

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
tests/cases/conformance/controlFlow/dependentDestructuredVariablesWithDefaults.ts	New regression test validating narrowing with required discriminants + defaults and guarding soundness boundaries.
tests/baselines/reference/dependentDestructuredVariablesWithDefaults.types	Baseline updated to reflect the new test’s expected type/narrowing results.
tests/baselines/reference/dependentDestructuredVariablesWithDefaults.symbols	Baseline updated to reflect expected symbol binding results for the new test.
tests/baselines/reference/dependentDestructuredVariablesWithDefaults.errors.txt	Baseline updated to capture expected errors for unsound narrowing cases.
src/compiler/checker.ts	Compiler change enabling dependent narrowing through destructured defaults when the default is provably never used.

[RyanCavanaugh]

The TypeScript repo is closed for development; PR should be in the typescript-go repo. Please see the top section of CONTRIBUTING.md and pinned issue #62963.