cmd/compile: don't crash on complex(0())

Fixes #17038.

Change-Id: Iaf6294361050040830af1d60cd48f263223d9356
Reviewed-on: https://go-review.googlesource.com/28966
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1397,6 +1397,10 @@ OpSwitch:
 			}
 
 			t := n.List.First().Left.Type
+			if !t.IsKind(TFUNC) {
+				// Bail. This error will be reported elsewhere.
+				return n
+			}
 			if t.Results().NumFields() != 2 {
 				Yyerror("invalid operation: complex expects two arguments, %v returns %d results", n.List.First(), t.Results().NumFields())
 				n.Type = nil

test/fixedbugs/issue17038.go

+// errorcheck
+
+// Copyright 2016 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 main
+
+const A = complex(0()) // ERROR "cannot call non-function"