It looks like Clang and GCC reject unevaluated call expressions and unary & expressions, but other non-ODR uses go undetected.
(void) main; // no diagnosis
(void) & main; // diagnosis
Also, this ODR use which avoids function pointer conversion gets no diagnosis from either.
template< int (&)() >
struct s {};
s< main > q; // no diagnosis
GCC diagnoses this analogous case, but Clang does not; it happily generates the recursive program.
#include <iostream>
void f( int (&p)() ) { p(); }
int main() {
std::cout << "hello\n";
f( main ); // GCC diagnosis, not Clang.
}
For the record, the GCC manual says that -pedantic and -pedantic-errors are required to get ISO-required diagnoses that aren’t “gratuitous.” So it seems there’s no sense worrying what they allow in non-pedantic mode.