cmd/compile: improved error message when calling a shadowed builtin

Otherwise, the error can be confusing if one forgets or doesn't know
that the builtin is being shadowed, which is not common practice.

Fixes #22822.

Change-Id: I735393b5ce28cb83815a1c3f7cd2e7bb5080a32d
Reviewed-on: https://go-review.googlesource.com/97455Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -1250,7 +1250,15 @@ func typecheck1(n *Node, top int) *Node {
 		default:
 			n.Op = OCALLFUNC
 			if t.Etype != TFUNC {
-				yyerror("cannot call non-function %v (type %v)", l, t)
+				name := l.String()
+				if isBuiltinFuncName(name) {
+					// be more specific when the function
+					// name matches a predeclared function
+					yyerror("cannot call non-function %s (type %v), declared at %s",
+						name, t, linestr(l.Name.Defn.Pos))
+				} else {
+					yyerror("cannot call non-function %s (type %v)", name, t)
+				}
 				n.Type = nil
 				return n
 			}

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

@@ -65,6 +65,17 @@ var builtinFuncs = [...]struct {
 	{"recover", ORECOVER},
 }
 
+// isBuiltinFuncName reports whether name matches a builtin function
+// name.
+func isBuiltinFuncName(name string) bool {
+	for _, fn := range builtinFuncs {
+		if fn.name == name {
+			return true
+		}
+	}
+	return false
+}
+
 var unsafeFuncs = [...]struct {
 	name string
 	op   Op

test/fixedbugs/issue22822.go

+// 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.
+
+// Check that calling a function shadowing a built-in provides a good
+// error message.
+
+package main
+
+func F() {
+	slice := []int{1, 2, 3}
+	len := int(2)
+	println(len(slice)) // ERROR "cannot call non-function len .type int., declared at"
+}