feat: restore PatchInstanceMethod (#20)

* restore PatchInstanceMethod
* fix global & restore test
* fix test

monkey.go

@@ -56,6 +56,20 @@ func Patch(target, replacement interface{}, opts ...Option) *PatchGuard {
 	return &PatchGuard{t, r, o}
 }
 
+// PatchInstanceMethod replaces an instance method methodName for the type target with replacement
+// Replacement should expect the receiver (of type target) as the first argument
+func PatchInstanceMethod(target reflect.Type, methodName string, replacement interface{}) *PatchGuard {
+	m, ok := target.MethodByName(methodName)
+	if !ok {
+		panic(fmt.Sprintf("unknown method %s", methodName))
+	}
+
+	o := &opt{global: true}
+	r := reflect.ValueOf(replacement)
+	patchValue(m.Func, r, o)
+	return &PatchGuard{m.Func, r, o}
+}
+
 // See reflect.Value
 type value struct {
 	_   uintptr

monkey_test.go

@@ -90,9 +90,9 @@ func (f *f) No() bool { return false }
 func TestOnInstanceMethod(t *testing.T) {
 	i := &f{}
 	assert(t, !i.No())
-	monkey.Patch((*f).No, func(_ *f) bool { return true })
+	pg := monkey.PatchInstanceMethod(reflect.TypeOf(i), "No", func(_ *f) bool { return true })
 	assert(t, i.No())
-	assert(t, monkey.UnpatchInstanceMethod(reflect.TypeOf(i), "No"))
+	pg.Unpatch()
 	assert(t, !i.No())
 }