Fix switch-true case-true narrowing to behave equivalently to default case

src/compiler/binder.ts

@@ -1718,7 +1718,8 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
         preSwitchCaseFlow = currentFlow;
         bind(node.caseBlock);
         addAntecedent(postSwitchLabel, currentFlow);
-        const hasDefault = forEach(node.caseBlock.clauses, c => c.kind === SyntaxKind.DefaultClause);
+        const hasDefault = forEach(node.caseBlock.clauses, c => c.kind === SyntaxKind.DefaultClause) ||
+            (skipParentheses(node.expression).kind === SyntaxKind.TrueKeyword && node.caseBlock.clauses.some(clause => clause.kind === SyntaxKind.CaseClause && skipParentheses(clause.expression).kind === SyntaxKind.TrueKeyword));
         // We mark a switch statement as possibly exhaustive if it has no default clause and if all
         // case clauses have unreachable end points (e.g. they all return). Note, we no longer need
         // this property in control flow analysis, it's there only for backwards compatibility.

tests/baselines/reference/narrowByClauseExpressionInSwitchTrue11.symbols

@@ -0,0 +1,46 @@
+//// [tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts] ////
+
+=== narrowByClauseExpressionInSwitchTrue11.ts ===
+function uhoh(input: string) : "A"|"B"|"C" {
+>uhoh : Symbol(uhoh, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 0))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))
+
+    switch (true) {
+        case /a/.test(input):
+>/a/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))
+
+            return "A";
+        case /b/.test(input):
+>/b/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 0, 14))
+
+            return "B";
+        case true:
+            return "C";
+    }
+}
+
+function uhoh1(input: string) : "A"|"B"|"C" {
+>uhoh1 : Symbol(uhoh1, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 9, 1))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))
+
+    switch ((true)) {
+        case /a/.test(input):
+>/a/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))
+
+            return "A";
+        case /b/.test(input):
+>/b/.test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>test : Symbol(RegExp.test, Decl(lib.es5.d.ts, --, --))
+>input : Symbol(input, Decl(narrowByClauseExpressionInSwitchTrue11.ts, 11, 15))
+
+            return "B";
+        case (true):
+            return "C";
+    }
+}

tests/baselines/reference/narrowByClauseExpressionInSwitchTrue11.types

@@ -0,0 +1,110 @@
+//// [tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts] ////
+
+=== narrowByClauseExpressionInSwitchTrue11.ts ===
+function uhoh(input: string) : "A"|"B"|"C" {
+>uhoh : (input: string) => "A" | "B" | "C"
+>     : ^     ^^      ^^^^^               
+>input : string
+>      : ^^^^^^
+
+    switch (true) {
+>true : true
+>     : ^^^^
+
+        case /a/.test(input):
+>/a/.test(input) : boolean
+>                : ^^^^^^^
+>/a/.test : (string: string) => boolean
+>         : ^      ^^      ^^^^^       
+>/a/ : RegExp
+>    : ^^^^^^
+>test : (string: string) => boolean
+>     : ^      ^^      ^^^^^       
+>input : string
+>      : ^^^^^^
+
+            return "A";
+>"A" : "A"
+>    : ^^^
+
+        case /b/.test(input):
+>/b/.test(input) : boolean
+>                : ^^^^^^^
+>/b/.test : (string: string) => boolean
+>         : ^      ^^      ^^^^^       
+>/b/ : RegExp
+>    : ^^^^^^
+>test : (string: string) => boolean
+>     : ^      ^^      ^^^^^       
+>input : string
+>      : ^^^^^^
+
+            return "B";
+>"B" : "B"
+>    : ^^^
+
+        case true:
+>true : true
+>     : ^^^^
+
+            return "C";
+>"C" : "C"
+>    : ^^^
+    }
+}
+
+function uhoh1(input: string) : "A"|"B"|"C" {
+>uhoh1 : (input: string) => "A" | "B" | "C"
+>      : ^     ^^      ^^^^^               
+>input : string
+>      : ^^^^^^
+
+    switch ((true)) {
+>(true) : true
+>       : ^^^^
+>true : true
+>     : ^^^^
+
+        case /a/.test(input):
+>/a/.test(input) : boolean
+>                : ^^^^^^^
+>/a/.test : (string: string) => boolean
+>         : ^      ^^      ^^^^^       
+>/a/ : RegExp
+>    : ^^^^^^
+>test : (string: string) => boolean
+>     : ^      ^^      ^^^^^       
+>input : string
+>      : ^^^^^^
+
+            return "A";
+>"A" : "A"
+>    : ^^^
+
+        case /b/.test(input):
+>/b/.test(input) : boolean
+>                : ^^^^^^^
+>/b/.test : (string: string) => boolean
+>         : ^      ^^      ^^^^^       
+>/b/ : RegExp
+>    : ^^^^^^
+>test : (string: string) => boolean
+>     : ^      ^^      ^^^^^       
+>input : string
+>      : ^^^^^^
+
+            return "B";
+>"B" : "B"
+>    : ^^^
+
+        case (true):
+>(true) : true
+>       : ^^^^
+>true : true
+>     : ^^^^
+
+            return "C";
+>"C" : "C"
+>    : ^^^
+    }
+}

tests/cases/compiler/narrowByClauseExpressionInSwitchTrue11.ts

@@ -0,0 +1,24 @@
+// @strict: true
+// @noEmit: true
+
+function uhoh(input: string) : "A"|"B"|"C" {
+    switch (true) {
+        case /a/.test(input):
+            return "A";
+        case /b/.test(input):
+            return "B";
+        case true:
+            return "C";
+    }
+}
+
+function uhoh1(input: string) : "A"|"B"|"C" {
+    switch ((true)) {
+        case /a/.test(input):
+            return "A";
+        case /b/.test(input):
+            return "B";
+        case (true):
+            return "C";
+    }
+}