Fix typeof switch narrowing broken by case "" clauses

internal/checker/flow.go

@@ -1967,19 +1967,19 @@ func (c *Checker) getSwitchClauseTypeOfWitnesses(node *ast.Node) []string {
 	links := c.switchStatementLinks.Get(node)
 	if !links.witnessesComputed {
 		clauses := node.AsSwitchStatement().CaseBlock.AsCaseBlock().Clauses.Nodes
-		witnesses := make([]string, len(clauses))
-		for i, clause := range clauses {
-			if clause.Kind == ast.KindCaseClause {
-				var text string
-				if ast.IsStringLiteralLike(clause.Expression()) {
-					text = clause.Expression().Text()
-				}
-				if text == "" {
-					witnesses = nil
-					break
-				}
-				if !slices.Contains(witnesses, text) {
-					witnesses[i] = text
+		// Return nil if one or more case clause expressions are not string literals.
+		hasNonStringLiteral := core.Some(clauses, func(clause *ast.Node) bool {
+			return clause.Kind == ast.KindCaseClause && !ast.IsStringLiteralLike(clause.Expression())
+		})
+		var witnesses []string
+		if !hasNonStringLiteral {
+			witnesses = make([]string, len(clauses))
+			for i, clause := range clauses {
+				if clause.Kind == ast.KindCaseClause {
+					text := clause.Expression().Text()
+					if text != "" && !slices.Contains(witnesses, text) {
+						witnesses[i] = text
+					}
 				}
 			}
 		}

testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.errors.txt

@@ -0,0 +1,15 @@
+typeofSwitchEmptyStringCase.ts(3,10): error TS2678: Type '""' is not comparable to type '"bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"'.
+
+
+==== typeofSwitchEmptyStringCase.ts (1 errors) ====
+    function f(x: string | number) {
+      switch (typeof x) {
+        case "":
+             ~~
+!!! error TS2678: Type '""' is not comparable to type '"bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"'.
+        case "string":
+          x.charAt(0);
+          break;
+      }
+    }
+

testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.js

@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/typeofSwitchEmptyStringCase.ts] ////
+
+//// [typeofSwitchEmptyStringCase.ts]
+function f(x: string | number) {
+  switch (typeof x) {
+    case "":
+    case "string":
+      x.charAt(0);
+      break;
+  }
+}
+
+
+//// [typeofSwitchEmptyStringCase.js]
+"use strict";
+function f(x) {
+    switch (typeof x) {
+        case "":
+        case "string":
+            x.charAt(0);
+            break;
+    }
+}

testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.symbols

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/typeofSwitchEmptyStringCase.ts] ////
+
+=== typeofSwitchEmptyStringCase.ts ===
+function f(x: string | number) {
+>f : Symbol(f, Decl(typeofSwitchEmptyStringCase.ts, 0, 0))
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 0, 11))
+
+  switch (typeof x) {
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 0, 11))
+
+    case "":
+    case "string":
+      x.charAt(0);
+>x.charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 0, 11))
+>charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+
+      break;
+  }
+}
+

testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.types

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/typeofSwitchEmptyStringCase.ts] ////
+
+=== typeofSwitchEmptyStringCase.ts ===
+function f(x: string | number) {
+>f : (x: string | number) => void
+>x : string | number
+
+  switch (typeof x) {
+>typeof x : "bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"
+>x : string | number
+
+    case "":
+>"" : ""
+
+    case "string":
+>"string" : "string"
+
+      x.charAt(0);
+>x.charAt(0) : string
+>x.charAt : (pos: number) => string
+>x : string
+>charAt : (pos: number) => string
+>0 : 0
+
+      break;
+  }
+}
+

testdata/tests/cases/compiler/typeofSwitchEmptyStringCase.ts

@@ -0,0 +1,10 @@
+// @strict: true
+
+function f(x: string | number) {
+  switch (typeof x) {
+    case "":
+    case "string":
+      x.charAt(0);
+      break;
+  }
+}