cmd/gc: give an error if only one argument is given to complex()

Fixes #8501

Change-Id: I0dbbdded7f7924351c3d1841d60cb5c934b295b7
Reviewed-on: https://go-review.googlesource.com/2143Reviewed-by: Chris Manghane <cmang@golang.org>

src/cmd/gc/typecheck.c

@@ -1233,6 +1233,10 @@ reswitch:
 		ok |= Erv;
 		if(count(n->list) == 1) {
 			typechecklist(n->list, Efnstruct);
+			if(n->list->n->op != OCALLFUNC && n->list->n->op != OCALLMETH) {
+				yyerror("invalid operation: complex expects two arguments");
+				goto error;
+			}
 			t = n->list->n->left->type;
 			if(t->outtuple != 2) {
 				yyerror("invalid operation: complex expects two arguments, %N returns %d results", n->list->n, t->outtuple);

test/fixedbugs/issue8501.go

+// errorcheck
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T struct {
+	f float64
+}
+
+var t T
+
+func F() {
+	_ = complex(1.0) // ERROR "invalid operation|not enough arguments"
+	_ = complex(t.f) // ERROR "invalid operation|not enough arguments"
+}