gc: handle function calls in arguments to builtin complex operations.

Fixes #2582

R=rsc
CC=golang-dev
https://golang.org/cl/5574044

src/cmd/gc/cplx.c

@@ -204,6 +204,8 @@ complexgen(Node *n, Node *res)
 	case OIND:
 	case ONAME:	// PHEAP or PPARAMREF var
 	case OCALLFUNC:
+	case OCALLMETH:
+	case OCALLINTER:
 		igen(n, &n1, res);
 		complexmove(&n1, res);
 		regfree(&n1);

test/fixedbugs/bug401.go

@@ -0,0 +1,29 @@
+// $G $D/$F.go || echo "Bug398"
+
+// Copyright 2011 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.
+
+// Issue 2582
+package foo
+    
+type T struct {}
+func (T) cplx() complex128 {
+	for false {}  // avoid inlining
+	return complex(1,0)
+}
+
+type I interface {
+	cplx() complex128
+}
+
+func f(e float32, t T) {
+
+    	_ = real(t.cplx())
+    	_ = imag(t.cplx())
+
+	var i I
+	i = t
+    	_ = real(i.cplx())
+    	_ = imag(i.cplx())
+}