compiler: set varargs correctly for type of method expression

Fixes golang/go#41737

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/258977

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c9c084bce713e258721e12041a351ec8ad33ad17
+801c458a562d22260ff176c26d65639dd32c8a90
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -5350,8 +5350,12 @@ Function_type::copy_with_receiver_as_param(bool want_pointer_receiver) const
 	   ++p)
 	new_params->push_back(*p);
     }
-  return Type::make_function_type(NULL, new_params, this->results_,
-				  this->location_);
+  Function_type* ret = Type::make_function_type(NULL, new_params,
+						this->results_,
+						this->location_);
+  if (this->is_varargs_)
+    ret->set_is_varargs();
+  return ret;
 }
 
 // Make a copy of a function type ignoring any receiver and adding a

libgo/go/reflect/all_test.go

@@ -2396,8 +2396,14 @@ func TestVariadicMethodValue(t *testing.T) {
 	points := []Point{{20, 21}, {22, 23}, {24, 25}}
 	want := int64(p.TotalDist(points[0], points[1], points[2]))
 
+	// Variadic method of type.
+	tfunc := TypeOf((func(Point, ...Point) int)(nil))
+	if tt := TypeOf(p).Method(4).Type; tt != tfunc {
+		t.Errorf("Variadic Method Type from TypeOf is %s; want %s", tt, tfunc)
+	}
+
 	// Curried method of value.
-	tfunc := TypeOf((func(...Point) int)(nil))
+	tfunc = TypeOf((func(...Point) int)(nil))
 	v := ValueOf(p).Method(4)
 	if tt := v.Type(); tt != tfunc {
 		t.Errorf("Variadic Method Type is %s; want %s", tt, tfunc)