cmd/compile: don't crash reporting misuse of shadowed built-in function

The existing implementation causes a compiler panic if a function parameter shadows a built-in function, and then calling that shadowed name.

Fixes golang#27356
Change-Id: I1ffb6dc01e63c7f499e5f6f75f77ce2318f35bcd
Reviewed-on: https://go-review.googlesource.com/132876
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1263,7 +1263,7 @@ func typecheck1(n *Node, top int) *Node {
 			n.Op = OCALLFUNC
 			if t.Etype != TFUNC {
 				name := l.String()
-				if isBuiltinFuncName(name) {
+				if isBuiltinFuncName(name) && l.Name.Defn != nil {
 					// be more specific when the function
 					// name matches a predeclared function
 					yyerror("cannot call non-function %s (type %v), declared at %s",

test/fixedbugs/issue27356.go

@@ -0,0 +1,19 @@
+// errorcheck
+
+// Copyright 2018 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 27356: function parameter hiding built-in function results in compiler crash
+
+package p
+
+var a = []int{1,2,3}
+
+func _(len int) {
+	_ =  len(a) // ERROR "cannot call non-function"
+}
+
+var cap = false
+var _ = cap(a) // ERROR "cannot call non-function"
+