allow narrowing for any left-hand in operand

src/compiler/binder.ts

@@ -916,8 +916,8 @@ namespace ts {
             return isTypeOfExpression(expr1) && isNarrowableOperand(expr1.expression) && isStringLiteralLike(expr2);
         }
 
-        function isNarrowableInOperands(left: Expression, right: Expression) {
-            return isNarrowingExpression(right) && (isIdentifier(left) || isStringLiteralLike(left));
+        function isNarrowableInRightOperand(expr: Expression) {
+            return isNarrowingExpression(expr);
         }
 
         function isNarrowingBinaryExpression(expr: BinaryExpression) {
@@ -936,7 +936,7 @@ namespace ts {
                 case SyntaxKind.InstanceOfKeyword:
                     return isNarrowableOperand(expr.left);
                 case SyntaxKind.InKeyword:
-                    return isNarrowableInOperands(expr.left, expr.right);
+                    return isNarrowableInRightOperand(expr.right);
                 case SyntaxKind.CommaToken:
                     return isNarrowingExpression(expr.right);
             }

tests/baselines/reference/controlFlowForInStatement2.js

@@ -0,0 +1,37 @@
+//// [controlFlowForInStatement2.ts]
+const keywordA = 'a';
+const keywordB = 'b';
+
+type A = { [keywordA]: number };
+type B = { [keywordB]: string };
+
+declare const c: A | B;
+
+if ('a' in c) {
+    c; // narrowed to `A`
+}
+
+if (keywordA in c) {
+    c; // also narrowed to `A`
+}
+
+let stringB: string = 'b';
+
+if ((stringB as 'b') in c) {
+    c; // narrowed to `B`
+}
+
+
+//// [controlFlowForInStatement2.js]
+var keywordA = 'a';
+var keywordB = 'b';
+if ('a' in c) {
+    c; // narrowed to `A`
+}
+if (keywordA in c) {
+    c; // also narrowed to `A`
+}
+var stringB = 'b';
+if (stringB in c) {
+    c; // narrowed to `B`
+}

tests/baselines/reference/controlFlowForInStatement2.symbols

@@ -0,0 +1,48 @@
+=== tests/cases/conformance/controlFlow/controlFlowForInStatement2.ts ===
+const keywordA = 'a';
+>keywordA : Symbol(keywordA, Decl(controlFlowForInStatement2.ts, 0, 5))
+
+const keywordB = 'b';
+>keywordB : Symbol(keywordB, Decl(controlFlowForInStatement2.ts, 1, 5))
+
+type A = { [keywordA]: number };
+>A : Symbol(A, Decl(controlFlowForInStatement2.ts, 1, 21))
+>[keywordA] : Symbol([keywordA], Decl(controlFlowForInStatement2.ts, 3, 10))
+>keywordA : Symbol(keywordA, Decl(controlFlowForInStatement2.ts, 0, 5))
+
+type B = { [keywordB]: string };
+>B : Symbol(B, Decl(controlFlowForInStatement2.ts, 3, 32))
+>[keywordB] : Symbol([keywordB], Decl(controlFlowForInStatement2.ts, 4, 10))
+>keywordB : Symbol(keywordB, Decl(controlFlowForInStatement2.ts, 1, 5))
+
+declare const c: A | B;
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+>A : Symbol(A, Decl(controlFlowForInStatement2.ts, 1, 21))
+>B : Symbol(B, Decl(controlFlowForInStatement2.ts, 3, 32))
+
+if ('a' in c) {
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+
+    c; // narrowed to `A`
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+}
+
+if (keywordA in c) {
+>keywordA : Symbol(keywordA, Decl(controlFlowForInStatement2.ts, 0, 5))
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+
+    c; // also narrowed to `A`
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+}
+
+let stringB: string = 'b';
+>stringB : Symbol(stringB, Decl(controlFlowForInStatement2.ts, 16, 3))
+
+if ((stringB as 'b') in c) {
+>stringB : Symbol(stringB, Decl(controlFlowForInStatement2.ts, 16, 3))
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+
+    c; // narrowed to `B`
+>c : Symbol(c, Decl(controlFlowForInStatement2.ts, 6, 13))
+}
+

tests/baselines/reference/controlFlowForInStatement2.types

@@ -0,0 +1,55 @@
+=== tests/cases/conformance/controlFlow/controlFlowForInStatement2.ts ===
+const keywordA = 'a';
+>keywordA : "a"
+>'a' : "a"
+
+const keywordB = 'b';
+>keywordB : "b"
+>'b' : "b"
+
+type A = { [keywordA]: number };
+>A : A
+>[keywordA] : number
+>keywordA : "a"
+
+type B = { [keywordB]: string };
+>B : B
+>[keywordB] : string
+>keywordB : "b"
+
+declare const c: A | B;
+>c : A | B
+
+if ('a' in c) {
+>'a' in c : boolean
+>'a' : "a"
+>c : A | B
+
+    c; // narrowed to `A`
+>c : A
+}
+
+if (keywordA in c) {
+>keywordA in c : boolean
+>keywordA : "a"
+>c : A | B
+
+    c; // also narrowed to `A`
+>c : A
+}
+
+let stringB: string = 'b';
+>stringB : string
+>'b' : "b"
+
+if ((stringB as 'b') in c) {
+>(stringB as 'b') in c : boolean
+>(stringB as 'b') : "b"
+>stringB as 'b' : "b"
+>stringB : string
+>c : A | B
+
+    c; // narrowed to `B`
+>c : B
+}
+

tests/cases/conformance/controlFlow/controlFlowForInStatement2.ts

@@ -0,0 +1,21 @@
+const keywordA = 'a';
+const keywordB = 'b';
+
+type A = { [keywordA]: number };
+type B = { [keywordB]: string };
+
+declare const c: A | B;
+
+if ('a' in c) {
+    c; // narrowed to `A`
+}
+
+if (keywordA in c) {
+    c; // also narrowed to `A`
+}
+
+let stringB: string = 'b';
+
+if ((stringB as 'b') in c) {
+    c; // narrowed to `B`
+}