Fix getSwitchClauseTypeOfWitnesses

[ahejlsberg]

Simpler fix than #3934.

Fixes #3909.

[Copilot]

Pull request overview

Fixes a bug in getSwitchClauseTypeOfWitnesses where a case "" clause inside a switch (typeof x) aborted witness collection entirely, disabling all type narrowing for the switch. The previous code conflated "expression is not a string literal" with "expression is the empty string literal" since both produced text == "". The fix separates these two concerns: a non-string-literal expression aborts witness collection (as before), while an empty string literal is now treated as an empty witness slot, allowing narrowing to proceed via the other clauses.

Changes:

• Split the text == "" early-out into a non-string-literal check that nils witnesses, and a separate empty/deduplicated witness handling for string literal text.
• Added a compiler regression test typeofSwitchEmptyStringCase.ts with .types, .symbols, and .errors.txt baselines confirming x narrows to string and only the expected TS2678 error is reported.

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
internal/checker/flow.go	Separates non-string-literal abort from empty-string handling in switch typeof witness collection.
testdata/tests/cases/compiler/typeofSwitchEmptyStringCase.ts	New regression test for issue #3909.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.types	Baseline showing x correctly narrowed to string.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.symbols	Symbol baseline for the new test.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.errors.txt	Error baseline showing only the expected TS2678 diagnostic.