__func__ is not a keyword

This program does not compile..in gcc and clang
int __func__  i =1;
int main () {}
error: expected identifier...
Standard has..
"The identifier __func__ shall be implicitly declared by the translator as if, immediately following
the opening brace of each function definition, the declaration" ...
My understand is that __func__   is not a keyword and that is something defined inside the functions.. so I don’t know why gcc and clang complains in the file scope.