Fix logic error in ported code

[ahejlsberg]

Fixes #944.

[Copilot]

Pull Request Overview

This PR adds a new exhaustive‐narrowing switch test for discriminant properties and corrects the logic in the type‐narrowing flow to properly handle missing discriminant types.

• Introduce switchExhaustiveNarrowing.ts to verify exhaustive checks including undefined
• Update baselines for .types and .symbols to reflect the new test
• Change narrowTypeByDiscriminant in flow.go to replace a nil check with an explicit fallback to unknownType

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
testdata/tests/cases/compiler/switchExhaustiveNarrowing.ts	Add a test covering exhaustive switch on source?.type
testdata/baselines/reference/compiler/switchExhaustiveNarrowing.types	Update expected type printing for the new test
testdata/baselines/reference/compiler/switchExhaustiveNarrowing.symbols	Update symbol output for the new test
internal/checker/flow.go	Fix logic in narrowTypeByDiscriminant to handle nil via unknownType

Comments suppressed due to low confidence (1)

internal/checker/flow.go:720

• [nitpick] Add a comment explaining why we use unknownType as a fallback for missing discriminants, to clarify the intended behavior when the property does not exist.

return c.filterType(t, func(t *Type) bool {