go.tools/go/types: added TODO re: misleading error message

Fix not urgent.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/135810043

go/types/builtins.go

@@ -495,6 +495,10 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			check.invalidArg(x.pos(), "%s has no single field %s", base, sel)
 			return
 		case *Func:
+			// TODO(gri) Using derefStructPtr may result in methods being found
+			// that don't actually exist. An error either way, but the error
+			// message is confusing. See: http://play.golang.org/p/al75v23kUy ,
+			// but go/types reports: "invalid argument: x.m is a method value".
 			check.invalidArg(arg0.Pos(), "%s is a method value", arg0)
 			return
 		}