Non-arithmetic format_as is broken with fmt/core.h

[jeremy-rifkin]

#3959 suggests that format_as should be supported with just fmt/core.h. I have hit a very surprising behavior where format_as returning a non-arithmetic type is completely broken unless including fmt/format.h: https://godbolt.org/z/344bznojf.

I see fmt/core.h is now just an alias for fmt/format.h on master so this should not reproduce currently, however, I want to flag this behavior in case it is unintentional and in case there are 10.x patch releases before the current core.h change is released.