Fix getSwitchClauseTypeOfWitnesses

internal/checker/flow.go

@@ -1970,15 +1970,11 @@ func (c *Checker) getSwitchClauseTypeOfWitnesses(node *ast.Node) []string {
 		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 == "" {
+				if !ast.IsStringLiteralLike(clause.Expression()) {
 					witnesses = nil
 					break
 				}
-				if !slices.Contains(witnesses, text) {
+				if text := clause.Expression().Text(); !slices.Contains(witnesses, text) {
 					witnesses[i] = text
 				}
 			}

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

@@ -0,0 +1,17 @@
+typeofSwitchEmptyStringCase.ts(5,10): error TS2678: Type '""' is not comparable to type '"bigint" | "boolean" | "function" | "number" | "object" | "string" | "symbol" | "undefined"'.
+
+
+==== typeofSwitchEmptyStringCase.ts (1 errors) ====
+    // https://github.com/microsoft/typescript-go/issues/3909
+
+    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.symbols

@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/typeofSwitchEmptyStringCase.ts] ////
+
+=== typeofSwitchEmptyStringCase.ts ===
+// https://github.com/microsoft/typescript-go/issues/3909
+
+function f(x: string | number) {
+>f : Symbol(f, Decl(typeofSwitchEmptyStringCase.ts, 0, 0))
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 2, 11))
+
+  switch (typeof x) {
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 2, 11))
+
+    case "":
+    case "string":
+      x.charAt(0);
+>x.charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+>x : Symbol(x, Decl(typeofSwitchEmptyStringCase.ts, 2, 11))
+>charAt : Symbol(String.charAt, Decl(lib.es5.d.ts, --, --))
+
+      break;
+  }
+}
+

testdata/baselines/reference/compiler/typeofSwitchEmptyStringCase.types

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/typeofSwitchEmptyStringCase.ts] ////
+
+=== typeofSwitchEmptyStringCase.ts ===
+// https://github.com/microsoft/typescript-go/issues/3909
+
+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 +1,12 @@
+// @noEmit: true
+
+// https://github.com/microsoft/typescript-go/issues/3909
+
+function f(x: string | number) {
+  switch (typeof x) {
+    case "":
+    case "string":
+      x.charAt(0);
+      break;
+  }
+}