Fix typeof switch narrowing broken by case "" clauses

[Copilot]

getSwitchClauseTypeOfWitnesses conflated empty string literal values (case "") with non-string-literal expressions — both produced text == "", which aborted witness collection entirely and disabled all type narrowing for the switch.

function f(x: string | number) {
  switch (typeof x) {
    case "":       // empty string — should be ignored for narrowing, not kill it
    case "string": // valid witness — should narrow x to string
      x.charAt(0); // ❌ incorrectly errors: "Property 'charAt' does not exist on type 'string | number'"
      break;
  }
}

• Separated the two concerns: non-string-literal case expressions are now checked upfront via core.Some, and empty string literals are treated as empty witnesses (no narrowing for that clause) rather than aborting the entire witness array — matching the TS reference implementation's text && !contains(witnesses, text) ? text : undefined logic.

[jakebailey]

@copilot The original code had const witnesses: (string | undefined)[] = [];; I'm not exactly sure that we won't have other errors related to this? Surely empty text will still cause problems as the slice is still of strings

[Copilot]

The []string with "" as the undefined equivalent is the pre-existing design — default clauses and duplicate case values already produced "" entries in the witnesses slice before my change. All three consumers guard against empty strings:

1. narrowTypeBySwitchOnTypeOf (line 1152): if text != "" → maps empty witnesses to neverType
2. getNotEqualFactsFromTypeofSwitch (line 1996): witness != "" → skips empty witnesses
3. computeExhaustiveSwitchStatement → delegates to getNotEqualFactsFromTypeofSwitch

So case "" now produces the same "" witness entry that default clauses and duplicates already do — meaning "no narrowing for this clause" — instead of aborting the entire witness array. The behavior matches the TS reference where undefined witnesses are simply skipped in all consuming code paths.

[Copilot]

Pull request overview

Fixes control-flow narrowing for switch (typeof x) when a clause uses an empty string literal (case ""), ensuring witness collection isn’t aborted and subsequent valid typeof cases still narrow correctly.

Changes:

• Updated getSwitchClauseTypeOfWitnesses to (1) bail out only when any case expression is not a string-literal-like node, and (2) treat "" as an empty witness rather than cancelling witness collection.
• Added a compiler test covering case "" followed by a valid case "string" and updated reference baselines to validate the narrowing behavior.

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
internal/checker/flow.go	Fixes witness collection for typeof switches so empty-string cases don’t disable narrowing for the entire switch.
testdata/tests/cases/compiler/typeofSwitchEmptyStringCase.ts	Adds a regression test for case "" in typeof switches.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.types	Verifies that x narrows to string inside the "string" case despite the earlier case "".
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.symbols	Captures expected symbol resolution for the new test.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.js	Captures expected emit output for the new test.
testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.errors.txt	Captures the expected comparability error for case "" while still allowing narrowing.