More exhaustive feature test macro checks

[joshessman-llnl]

This was discovered when using a compiler that supports if constepxr but not automatic return type deduction, which is used by the get function defined within this block. I was able to mitigate this by adding a check to make sure automatic return type deduction was also supported.

[alexezeder]

Do you still have access to this compiler? I'm asking because, as far as I can see, it won't pass the compile-test since there is the same check:

fmt/test/compile-test.cc

Line 79 in dcd282b

#ifdef __cpp_if_constexpr

and it's unmodified in this PR.
Also, there are other usages of this feature macro in compile.h:

fmt/include/fmt/compile.h

Line 159 in dcd282b

#ifdef __cpp_if_constexpr

fmt/include/fmt/compile.h

Line 538 in dcd282b

#ifdef __cpp_if_constexpr

that probably should also be changed since they depend on each other.

[joshessman-llnl]

Thanks for pointing this out! I was able to extend the conditional blocks mentioned above and ran into another bug in Intel (minimal example here) that I went ahead and added a patch for.

[vitaut]

Thanks for the PR. Looks good but please address minor inline comments.

[alexezeder]

I can also suggest regrouping changes to 2 commits differently than it's done now - the first one for all if constepxr changes, the second just for the Intel compiler bug, since these two fixes are unrelated.

[joshessman-llnl]

I can also suggest regrouping changes to 2 commits differently than it's done now - the first one for all if constepxr changes, the second just for the Intel compiler bug, since these two fixes are unrelated.

Good suggestion, the two changes have been moved into their own commits.