Pick into release-2.1: Make 'keyof T' a literal contextual type

src/compiler/checker.ts

@@ -14870,7 +14870,7 @@ namespace ts {
                     }
                     contextualType = apparentType;
                 }
-                return maybeTypeOfKind(contextualType, TypeFlags.Literal);
+                return maybeTypeOfKind(contextualType, (TypeFlags.Literal | TypeFlags.Index));
             }
             return false;
         }

tests/baselines/reference/keyofIsLiteralContexualType.errors.txt

@@ -0,0 +1,32 @@
+tests/cases/compiler/keyofIsLiteralContexualType.ts(5,9): error TS2322: Type '("a" | "b" | "c")[]' is not assignable to type 'keyof T[]'.
+  Type '"a" | "b" | "c"' is not assignable to type 'keyof T'.
+    Type '"a" | "b" | "c"' is not assignable to type '"a" | "b"'.
+      Type '"c"' is not assignable to type '"a" | "b"'.
+        Type '"c"' is not assignable to type 'keyof T'.
+          Type '"c"' is not assignable to type '"a" | "b"'.
+tests/cases/compiler/keyofIsLiteralContexualType.ts(13,11): error TS2339: Property 'b' does not exist on type 'Pick<{ a: number; b: number; c: number; }, "a" | "c">'.
+
+
+==== tests/cases/compiler/keyofIsLiteralContexualType.ts (2 errors) ====
+    // keyof T is a literal contextual type
+
+    function foo<T extends { a: string, b: string }>() {
+        let a: (keyof T)[] = ["a", "b"];
+        let b: (keyof T)[] = ["a", "b", "c"];
+            ~
+!!! error TS2322: Type '("a" | "b" | "c")[]' is not assignable to type 'keyof T[]'.
+!!! error TS2322:   Type '"a" | "b" | "c"' is not assignable to type 'keyof T'.
+!!! error TS2322:     Type '"a" | "b" | "c"' is not assignable to type '"a" | "b"'.
+!!! error TS2322:       Type '"c"' is not assignable to type '"a" | "b"'.
+!!! error TS2322:         Type '"c"' is not assignable to type 'keyof T'.
+!!! error TS2322:           Type '"c"' is not assignable to type '"a" | "b"'.
+    }
+
+    // Repro from #12455
+
+    declare function pick<T, K extends keyof T>(obj: T, propNames: K[]): Pick<T, K>;
+
+    let x = pick({ a: 10, b: 20, c: 30 }, ["a", "c"]);
+    let b = x.b;  // Error
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'Pick<{ a: number; b: number; c: number; }, "a" | "c">'.

tests/baselines/reference/keyofIsLiteralContexualType.js

@@ -0,0 +1,23 @@
+//// [keyofIsLiteralContexualType.ts]
+// keyof T is a literal contextual type
+
+function foo<T extends { a: string, b: string }>() {
+    let a: (keyof T)[] = ["a", "b"];
+    let b: (keyof T)[] = ["a", "b", "c"];
+}
+
+// Repro from #12455
+
+declare function pick<T, K extends keyof T>(obj: T, propNames: K[]): Pick<T, K>;
+
+let x = pick({ a: 10, b: 20, c: 30 }, ["a", "c"]);
+let b = x.b;  // Error
+
+//// [keyofIsLiteralContexualType.js]
+// keyof T is a literal contextual type
+function foo() {
+    var a = ["a", "b"];
+    var b = ["a", "b", "c"];
+}
+var x = pick({ a: 10, b: 20, c: 30 }, ["a", "c"]);
+var b = x.b; // Error

tests/cases/compiler/keyofIsLiteralContexualType.ts

@@ -0,0 +1,13 @@
+// keyof T is a literal contextual type
+
+function foo<T extends { a: string, b: string }>() {
+    let a: (keyof T)[] = ["a", "b"];
+    let b: (keyof T)[] = ["a", "b", "c"];
+}
+
+// Repro from #12455
+
+declare function pick<T, K extends keyof T>(obj: T, propNames: K[]): Pick<T, K>;
+
+let x = pick({ a: 10, b: 20, c: 30 }, ["a", "c"]);
+let b = x.b;  // Error