Add functions to list of valid AST nodes

In theory we need to check if the function does not have
an environment but the goal of the check is not to be precise,
but rather to catch obvious mistakes.

Author: josevalim

lib/elixir/src/elixir_quote.erl

@@ -271,7 +271,7 @@ valid_ast_list([]) -> true;
 valid_ast_list([Head | Tail]) -> valid_ast_elem(Head) andalso valid_ast_list(Tail);
 valid_ast_list(_Improper) -> false.
 
-valid_ast_elem(Expr) when is_list(Expr); is_atom(Expr); is_binary(Expr); is_number(Expr); is_pid(Expr) -> true;
+valid_ast_elem(Expr) when is_list(Expr); is_atom(Expr); is_binary(Expr); is_number(Expr); is_pid(Expr); is_function(Expr) -> true;
 valid_ast_elem({Left, Right}) -> valid_ast_elem(Left) andalso valid_ast_elem(Right);
 valid_ast_elem({Atom, Meta, Args}) when is_atom(Atom), is_list(Meta), is_atom(Args) orelse is_list(Args) -> true;
 valid_ast_elem({Call, Meta, Args}) when is_list(Meta), is_list(Args) -> shallow_valid_ast(Call);