Use comparability for discriminant properties when narrowing types for a default switch clause

[Copilot]

Port of TypeScript#61211.

Enum member types used as case expressions weren't properly narrowing discriminated unions in default clauses because the check used strict equality instead of type comparability. For example, AnimalType.cat (enum type) wasn't matched against "cat" (string literal type), so the union was never fully exhausted.

• Replace slices.Contains(switchTypes, u) with slices.ContainsFunc using isUnitType(st) && areTypesComparable(st, u) in narrowTypeBySwitchOnDiscriminantProperty

return !slices.ContainsFunc(switchTypes, func(st *Type) bool {
    return isUnitType(st) && c.areTypesComparable(st, u)
})

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[Copilot]

Pull request overview

Ports TypeScript#61211 to fix discriminated-union exhaustion in default switch clauses when case expressions are enum member types, by using type comparability rather than strict equality.

Changes:

• Update switch-based discriminant narrowing to use slices.ContainsFunc with areTypesComparable for membership checks.
• Refresh conformance baselines to reflect correct narrowing to never (and removal of the prior error output).
• Update baseline diff artifacts accordingly.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.types.diff	Baseline diff updated to reflect new narrowing behavior in default.
testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.types	Baseline updated: animal correctly narrows to never in default.
testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.errors.txt.diff	Baseline diff updated to show the error is no longer produced.
testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.errors.txt	Error baseline removed since narrowing now exhausts the union.
internal/checker/flow.go	Core logic change: replace strict-equality membership with comparability-based predicate.