Add test for formatting with no args

[yeswalrus]

Demonstrates a bug reported in #2039.

The code involved here is a bit beyond me & the error message produced is rather inscrutable

[alexezeder]

Looks like it's a GCC bug to me, because it's reproducible only on GCC with -O2+ optimization level and -Warray-bounds diagnostic enabled. I got this warning also on Compiler Explorer but only for ARM GCC, on the other hand, x86_64 GCC does not output any warning on Compiler Explorer, but of course it does that on GitHub Actions environment and also on my PC.
Somehow GCC manages to get a possibility of a negative value in id in this function during compilation:

fmt/include/fmt/format.h

Lines 3221 to 3225 in e737672

	FMT_CONSTEXPR const Char* on_format_specs(int id, const Char* begin,
	const Char*) {
	advance_to(context_, begin);
	return id < num_args ? parse_funcs_[id](context_) : begin;
	}

This happens probably because of some weird optimization as this function is not even invoked for the provided format string.
I managed to get rid of this warning by explicitly checking if the id value is negative then call std::abort() to guard that array subscript in on_format_specs function for negative values:

if (id < 0) std::abort();
return id < num_args ? parse_funcs_[id](context_) : begin;

But it can introduce a side-effect in runtime because of the additional check.

[vitaut]

Good catch, thanks! Worked around in 3551f5d.

[alexezeder]

@vitaut, just for information, it's not specific for gcc 10 only, but looks like for all gcc versions, at least for 9.2, 7.3

[vitaut]

Thanks for the info, @alexezeder.