Make Cond Matcher generic (#183)

`gomock.Cond` at the moment expects an matcher function with the
signature `func(x any) bool` requiring a manual cast to the expected
type. By making this matcher generic the callback function can be typed
removing the need for a manual cast to the expected type.

The change should be backwards compatible with existing tests using
`gomock.Cond`.

gomock/matchers.go

@@ -98,15 +98,19 @@ func (anyMatcher) String() string {
 	return "is anything"
 }
 
-type condMatcher struct {
-	fn func(x any) bool
+type condMatcher[T any] struct {
+	fn func(x T) bool
 }
 
-func (c condMatcher) Matches(x any) bool {
-	return c.fn(x)
+func (c condMatcher[T]) Matches(x any) bool {
+	typed, ok := x.(T)
+	if !ok {
+		return false
+	}
+	return c.fn(typed)
 }
 
-func (condMatcher) String() string {
+func (c condMatcher[T]) String() string {
 	return "adheres to a custom condition"
 }
 
@@ -339,9 +343,9 @@ func Any() Matcher { return anyMatcher{} }
 //
 // Example usage:
 //
-//	Cond(func(x any){return x.(int) == 1}).Matches(1) // returns true
-//	Cond(func(x any){return x.(int) == 2}).Matches(1) // returns false
-func Cond(fn func(x any) bool) Matcher { return condMatcher{fn} }
+//	Cond(func(x int){return x == 1}).Matches(1) // returns true
+//	Cond(func(x int){return x == 2}).Matches(1) // returns false
+func Cond[T any](fn func(x T) bool) Matcher { return condMatcher[T]{fn} }
 
 // AnyOf returns a composite Matcher that returns true if at least one of the
 // matchers returns true.

gomock/matchers_test.go

@@ -57,7 +57,9 @@ func TestMatchers(t *testing.T) {
 			[]e{[]string{"a", "b"}, A{"a", "b"}},
 			[]e{[]string{"a"}, A{"b"}},
 		},
-		{"test Cond", gomock.Cond(func(x any) bool { return x.(B).Name == "Dam" }), []e{B{Name: "Dam"}}, []e{B{Name: "Dave"}}},
+		{"test Cond", gomock.Cond(func(x B) bool { return x.Name == "Dam" }), []e{B{Name: "Dam"}}, []e{B{Name: "Dave"}}},
+		{"test Cond wrong type", gomock.Cond(func(x B) bool { return x.Name == "Dam" }), []e{B{Name: "Dam"}}, []e{"Dave"}},
+		{"test Cond any type", gomock.Cond(func(x any) bool { return x.(B).Name == "Dam" }), []e{B{Name: "Dam"}}, []e{B{Name: "Dave"}}},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

sample/user_test.go

@@ -193,12 +193,8 @@ func TestExpectCondForeignFour(t *testing.T) {
 	defer ctrl.Finish()
 
 	mockIndex := NewMockIndex(ctrl)
-	mockIndex.EXPECT().ForeignFour(gomock.Cond(func(x any) bool {
-		four, ok := x.(imp_four.Imp4)
-		if !ok {
-			return false
-		}
-		return four.Field == "Cool"
+	mockIndex.EXPECT().ForeignFour(gomock.Cond(func(x imp_four.Imp4) bool {
+		return x.Field == "Cool"
 	}))
 
 	mockIndex.ForeignFour(imp_four.Imp4{Field: "Cool"})