cmd/compile: compute number of arguments correctly

The outCount value includes a flag bit for dotdotdot.

If we have this count incorrect, then the offset for the
methodset *rtype are in the wrong place.

Fixes #14783

Change-Id: If5acb16af08d4ffe36c8c9ee389c32f2712ce757
Reviewed-on: https://go-review.googlesource.com/20566Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -1068,7 +1068,7 @@ ok:
 			ot += 4 // align for *rtype
 		}
 
-		dataAdd := (inCount + outCount) * Widthptr
+		dataAdd := (inCount + t.Outtuple) * Widthptr
 		ot = dextratype(s, ot, t, dataAdd)
 
 		// Array of rtype pointers follows funcType.

src/reflect/all_test.go

@@ -2410,6 +2410,17 @@ func TestEmbeddedMethods(t *testing.T) {
 	}
 }
 
+type FuncDDD func(...interface{}) error
+
+func (f FuncDDD) M() {}
+
+func TestNumMethodOnDDD(t *testing.T) {
+	rv := ValueOf((FuncDDD)(nil))
+	if n := rv.NumMethod(); n != 1 {
+		t.Fatalf("NumMethod()=%d, want 1", n)
+	}
+}
+
 func TestPtrTo(t *testing.T) {
 	var i int