Fix narrowing of destructured discriminated union parameters with defaults

[wolfe8105]

Fixes #50139 — first PR here, happy to adjust style/scope as needed.

src/compiler/checker.ts had two predicates that skipped narrowing for destructured bindings with defaults:

• getCandidateDiscriminantPropertyAccess
• getNarrowedTypeOfSymbol

Both guarded with !declaration.initializer. Removed that clause from both. Narrowing now works through defaulted bindings.

Declaration-time assignability of the default is checked by a separate pipeline, so foreign-type defaults still error at the binding.

Regression test added. npx hereby runtests shows no existing baselines change.

---

• There is an associated issue in the Backlog milestone (Discriminated union parameter destructuring doesn't work if the fields have defaults #50139)
• Code is up-to-date with the main branch
• You've successfully run hereby runtests locally
• There are new or updated unit tests validating the change

[Copilot]

Pull request overview

Note

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

Adds a compiler regression test for destructuring defaults in discriminated unions (issue #50139) and updates checker logic so narrowing works when binding elements/parameters have default initializers.

Changes:

• Add a new compiler test covering: (1) repro with defaults, (2) control without defaults, (3) soundness error case.
• Update checker.ts control-flow/narrowing logic to consider binding elements/parameters even when they have default initializers.
• Add new baseline outputs (.types, .symbols, .js, .errors.txt) for the test.

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
tests/cases/compiler/discriminatedUnionDestructuringWithDefaults.ts	New regression test for narrowing behavior with destructuring defaults in discriminated unions
tests/baselines/reference/discriminatedUnionDestructuringWithDefaults.types	Baseline types output for the new test
tests/baselines/reference/discriminatedUnionDestructuringWithDefaults.symbols	Baseline symbols output for the new test
tests/baselines/reference/discriminatedUnionDestructuringWithDefaults.js	Baseline JS emit output for the new test
tests/baselines/reference/discriminatedUnionDestructuringWithDefaults.errors.txt	Baseline expected error output for the soundness case
src/compiler/checker.ts	Adjust narrowing/reference logic to no longer exclude binding elements/parameters with initializers

Comments suppressed due to low confidence (1)

src/compiler/checker.ts:1

• This condition used to explicitly exclude binding elements with an initializer; removing that guard is subtle and easy to accidentally revert later. Consider adding a brief comment here explaining that binding element default initializers are intentionally included so discriminated-union narrowing works for destructured bindings with defaults (as covered by the new test).

[wolfe8105]

@microsoft-github-policy-service agree

[MartinJohns]

@wolfe8105 You should read this: #62963