reflect: add Type.Elem() and Type.Key() for Maps

src/reflect/type.go

@@ -481,13 +481,21 @@ func (t *rawType) elem() *rawType {
 	switch underlying.Kind() {
 	case Pointer:
 		return (*ptrType)(unsafe.Pointer(underlying)).elem
-	case Chan, Slice, Array:
+	case Chan, Slice, Array, Map:
 		return (*elemType)(unsafe.Pointer(underlying)).elem
-	default: // not implemented: Map
+	default:
-		panic("unimplemented: (reflect.Type).Elem()")
+		panic(&TypeError{"Elem"})
 	}
 }
 
+func (t *rawType) key() *rawType {
+	underlying := t.underlying()
+	if underlying.Kind() != Map {
+		panic(&TypeError{"Key"})
+	}
+	return (*mapType)(unsafe.Pointer(underlying)).key
+}
+
 // Field returns the type of the i'th field of this struct type. It panics if t
 // is not a struct type.
 func (t *rawType) Field(i int) StructField {
@@ -806,7 +814,7 @@ func (t *rawType) Name() string {
 }
 
 func (t *rawType) Key() Type {
-	panic("unimplemented: (reflect.Type).Key()")
+	return t.key()
 }
 
 func (t rawType) In(i int) Type {

src/reflect/value_test.go

@@ -30,3 +30,18 @@ func TestIndirectPointers(t *testing.T) {
 		t.Errorf("bad indirect array index via reflect")
 	}
 }
+
+func TestMap(t *testing.T) {
+
+	m := make(map[string]int)
+
+	mtyp := TypeOf(m)
+
+	if got, want := mtyp.Key().Kind().String(), "string"; got != want {
+		t.Errorf("m.Type().Key().String()=%q, want %q", got, want)
+	}
+
+	if got, want := mtyp.Elem().Kind().String(), "int"; got != want {
+		t.Errorf("m.Elem().String()=%q, want %q", got, want)
+	}
+}