gcc warn C++ implicit enum conversion

[Jonny Grant <jg@xxxxxxxx>]

Hi
Is there any way to warn on enum conversion to int? I tried few options, 
but no warning seems possible.

Cheers, Jonny

//-O2 -Wall -Wextra -Wconversion -Werror

#include <cstddef>
typedef enum
{
    a = -1
} a_t;

a_t f()
{
    return a;
}

int main()
{
    int b = f();
    return b;
}