reflect: fix variadic arg for funcs created by MakeFunc.

Short circuit for calling values funcs by MakeFunc was placed
before variadic arg rearrangement code in reflect.call.
Fixes #7534.

LGTM=khr
R=golang-codereviews, bradfitz, khr, rsc
CC=golang-codereviews
https://golang.org/cl/75370043

src/pkg/reflect/all_test.go

@@ -1512,6 +1512,23 @@ func TestMakeFuncInterface(t *testing.T) {
 	}
 }
 
+func TestMakeFuncVariadic(t *testing.T) {
+	// Test that variadic arguments are packed into a slice and passed as last arg
+	fn := func(_ int, is ...int) []int { return nil }
+	fv := MakeFunc(TypeOf(fn), func(in []Value) []Value { return in[1:2] })
+	ValueOf(&fn).Elem().Set(fv)
+
+	r := fv.Call([]Value{ValueOf(1), ValueOf(2), ValueOf(3)})[0].Interface().([]int)
+	if r[0] != 2 || r[1] != 3 {
+		t.Errorf("Call returned [%v, %v]; want 2, 3", r[0], r[1])
+	}
+
+	r = fv.CallSlice([]Value{ValueOf(1), ValueOf([]int{2, 3})})[0].Interface().([]int)
+	if r[0] != 2 || r[1] != 3 {
+		t.Errorf("Call returned [%v, %v]; want 2, 3", r[0], r[1])
+	}
+}
+
 type Point struct {
 	x, y int
 }

src/pkg/reflect/value.go

@@ -453,17 +453,6 @@ func (v Value) call(op string, in []Value) []Value {
 		panic("reflect.Value.Call: call of nil function")
 	}
 
-	// If target is makeFuncStub, short circuit the unpack onto stack /
-	// pack back into []Value for the args and return values.  Just do the
-	// call directly.
-	// We need to do this here because otherwise we have a situation where
-	// reflect.callXX calls makeFuncStub, neither of which knows the
-	// layout of the args.  That's bad for precise gc & stack copying.
-	x := (*makeFuncImpl)(fn)
-	if x.code == makeFuncStubCode {
-		return x.fn(in)
-	}
-
 	isSlice := op == "CallSlice"
 	n := t.NumIn()
 	if isSlice {
@@ -521,6 +510,17 @@ func (v Value) call(op string, in []Value) []Value {
 	}
 	nout := t.NumOut()
 
+	// If target is makeFuncStub, short circuit the unpack onto stack /
+	// pack back into []Value for the args and return values.  Just do the
+	// call directly.
+	// We need to do this here because otherwise we have a situation where
+	// reflect.callXX calls makeFuncStub, neither of which knows the
+	// layout of the args.  That's bad for precise gc & stack copying.
+	x := (*makeFuncImpl)(fn)
+	if x.code == makeFuncStubCode {
+		return x.fn(in)
+	}
+
 	// If the target is methodValueCall, do its work here: add the receiver
 	// argument and call the real target directly.
 	// We need to do this here because otherwise we have a situation where