Allow AssignableToTypeOf reflect.Type (#365)

Fixes #320

gomock/matchers.go

@@ -244,6 +244,12 @@ func Not(x interface{}) Matcher {
 //   var s fmt.Stringer = &bytes.Buffer{}
 //   AssignableToTypeOf(s).Matches(time.Second) // returns true
 //   AssignableToTypeOf(s).Matches(99) // returns false
+//
+//   var ctx = reflect.TypeOf((*context.Context)).Elem()
+//   AssignableToTypeOf(ctx).Matches(context.Background()) // returns true
 func AssignableToTypeOf(x interface{}) Matcher {
+	if xt, ok := x.(reflect.Type); ok {
+		return assignableToTypeOfMatcher{xt}
+	}
 	return assignableToTypeOfMatcher{reflect.TypeOf(x)}
 }

gomock/matchers_test.go

@@ -17,7 +17,9 @@
 package gomock_test
 
 import (
+	"context"
 	"errors"
+	"reflect"
 	"testing"
 
 	"github.com/golang/mock/gomock"
@@ -123,4 +125,14 @@ func TestAssignableToTypeOfMatcher(t *testing.T) {
 	if match := gomock.AssignableToTypeOf(&Dog{}).Matches(&Dog{Breed: "pug", Name: "Fido"}); !match {
 		t.Errorf(`AssignableToTypeOf(&Dog{}) should match &Dog{Breed: "pug", Name: "Fido"}`)
 	}
+
+	ctxInterface := reflect.TypeOf((*context.Context)(nil)).Elem()
+	if match := gomock.AssignableToTypeOf(ctxInterface).Matches(context.Background()); !match {
+		t.Errorf(`AssignableToTypeOf(context.Context) should not match context.Background()`)
+	}
+
+	ctxWithValue := context.WithValue(context.Background(), "key", "val")
+	if match := gomock.AssignableToTypeOf(ctxInterface).Matches(ctxWithValue); !match {
+		t.Errorf(`AssignableToTypeOf(context.Context) should not match ctxWithValue`)
+	}
 }