Fix compilation error if PACKED is defined before including FMT

[glebm]

Reduces the chance of conflicts between a macro declaration and the template argument name by using CamelCase (PACKED -> IsPacked)

This improves compatibility with oddball SDKs, such as the 3DS libctru.

Fixes #3632

[vitaut]

Thanks for the PR but this should be fixed in libctru by prefixing the macro. We shouldn't be trying to avoid conflicts with poorly named macros from random third-party libraries. In the meantime you can undefine the problematic macro yourself after including the headers that define it.

[glebm]

Philosophically correct but why name a template parameter in all caps? This style is usually used for C macros and enums and doesn't seem consistent with the rest of libfmt anyway

[vitaut]

Non-type template parameters normally follow this style.

[ne-lu]

For what it's worth, my company also defines a PACKED macro. It's a fairly common alias for __attribute__((PACKED)), you see it a lot in embedded contexts for example.

Would you be open to changing this parameter name to IS_PACKED? There's already IS_CONSTEXPR in a few places, so it's still consistent.

You're right that it isn't this library's responsibility to ensure that your names don't conflict with other people's macros, but PACKED happens to be a fairly common macro people define, so it could save a lot of headaches for people.

[lapointexavier]

I would like to second that, ‘PACKED’ is also a bit too common and it’s causing headaches trying to upgrade fmt. Would you reconsider @vitaut ? If not we’ll find a solution but it seems like a small lift if this PR is all it takes.

[vitaut]

We don't try to workaround conflicts with poorly written macros unless they are part of system APIs but this particular one is no longer there for other reasons.