Problems building with not that old gcc versions since Boost 1.73

Since https://github.com/boostorg/safe_numerics/commit/937928693d40564a72fd48f63039040908098132, building

```
#include
#include
#include

template
struct fmt::formatter<
boost::safe_numerics::safe_base<
Stored, Min, Max, P, E
>
> : fmt::ostream_formatter
{
};

void la()
{
fmt::print("{}", boost::safe_numerics::safe{ 4 });
}

```

with gcc 8.2-10.4 with `-std=gnu++17 -Os -Wall` fails. https://godbolt.org/z/Goc3er7vc

It's clearly a gcc bug, so it would be fair game to ignore it. But it affects Ubuntu 20.04.