Addressing CR feedback.

Adding comment to explain source type instantiation.
Adding a test case.

tests/baselines/reference/contextualSignatureInstantiation3.js

@@ -0,0 +1,42 @@
+//// [contextualSignatureInstantiation3.ts]
+function map<T, U>(items: T[], f: (x: T) => U): U[]{
+    return items.map(f);
+}
+
+function identity<T>(x: T) {
+    return x;
+}
+
+function singleton<T>(x: T) {
+    return [x];
+}
+
+var xs = [1, 2, 3];
+
+// Have compiler check that we get the correct types
+var v1: number[];
+var v1 = xs.map(identity);      // Error if not number[]
+var v1 = map(xs, identity);     // Error if not number[]
+
+var v2: number[][];         
+var v2 = xs.map(singleton);     // Error if not number[][]
+var v2 = map(xs, singleton);    // Error if not number[][]
+
+
+//// [contextualSignatureInstantiation3.js]
+function map(items, f) {
+    return items.map(f);
+}
+function identity(x) {
+    return x;
+}
+function singleton(x) {
+    return [x];
+}
+var xs = [1, 2, 3];
+var v1;
+var v1 = xs.map(identity);
+var v1 = map(xs, identity);
+var v2;
+var v2 = xs.map(singleton);
+var v2 = map(xs, singleton);

tests/cases/compiler/contextualSignatureInstantiation3.ts

@@ -0,0 +1,22 @@
+function map<T, U>(items: T[], f: (x: T) => U): U[]{
+    return items.map(f);
+}
+
+function identity<T>(x: T) {
+    return x;
+}
+
+function singleton<T>(x: T) {
+    return [x];
+}
+
+var xs = [1, 2, 3];
+
+// Have compiler check that we get the correct types
+var v1: number[];
+var v1 = xs.map(identity);      // Error if not number[]
+var v1 = map(xs, identity);     // Error if not number[]
+
+var v2: number[][];         
+var v2 = xs.map(singleton);     // Error if not number[][]
+var v2 = map(xs, singleton);    // Error if not number[][]