From cdda5dfd61fd0b3a7288619895ec3f85ebec1c70 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 23 Nov 2016 18:22:57 -0800
Subject: [PATCH 1/2] keyof T is a literal contextual type

---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6d5a8b74cb3bd..f0c6e4edb42a8 100644
--- a/src/compiler/checker.ts
+++ b/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;
         }

From 2cec4c5ebb2cfde2119f04b299fc91fdf05f183c Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 23 Nov 2016 18:28:59 -0800
Subject: [PATCH 2/2] Add regression test

---
 .../keyofIsLiteralContexualType.errors.txt    | 32 +++++++++++++++++++
 .../reference/keyofIsLiteralContexualType.js  | 23 +++++++++++++
 .../compiler/keyofIsLiteralContexualType.ts   | 13 ++++++++
 3 files changed, 68 insertions(+)
 create mode 100644 tests/baselines/reference/keyofIsLiteralContexualType.errors.txt
 create mode 100644 tests/baselines/reference/keyofIsLiteralContexualType.js
 create mode 100644 tests/cases/compiler/keyofIsLiteralContexualType.ts

diff --git a/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt b/tests/baselines/reference/keyofIsLiteralContexualType.errors.txt
new file mode 100644
index 0000000000000..8424d528b2292
--- /dev/null
+++ b/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">'.
\ No newline at end of file
diff --git a/tests/baselines/reference/keyofIsLiteralContexualType.js b/tests/baselines/reference/keyofIsLiteralContexualType.js
new file mode 100644
index 0000000000000..5325b97617483
--- /dev/null
+++ b/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
diff --git a/tests/cases/compiler/keyofIsLiteralContexualType.ts b/tests/cases/compiler/keyofIsLiteralContexualType.ts
new file mode 100644
index 0000000000000..dd14de9228947
--- /dev/null
+++ b/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
\ No newline at end of file