clang-tidy failure on provided example

[jefftrull]

The current main will fail clang-tidy (but pass clang and gcc compilation) on a simple example from the docs:

#include <stdx/ct_format.hpp>
int main() {
    auto s = stdx::ct_format<"Hello {} {}">(CX_VALUE(42), 17);
}

The errors are:

stdx/ct_format.hpp:348:28: error: constexpr variable 'sz' must be initialized by a constant expression [clang-diagnostic-error]
stdx/ct_format.hpp:349:34: error: no matching function for call to 'perform_format' [clang-diagnostic-error]

I believe the source of this problem is a commit in the fmt library as described in this issue.

[jefftrull]

I managed to create a reproducer in fmt, issue here