Bareflank / bsl

First of all, thanks for a good library.

I had tested this library for use on armv7m architecture but sadly the Rust friendly design breaks it since uintmx is 64 bit (unsigned long long) on 32 bit architecture. uintptr_t and size_t is always a safe bet across architectures. I'm now considering what to do, but i found a few things to share, if there are any interest in GCC support.

Use: ´#pragma GCC diagnostic´ instead. since it works in both gcc and clang.
No -Wno-invalid-constexpr exists in gcc. I can currently not find an alternative, it might be a problem.
Return value in void functions? GCC don't like it.

bsl/include/bsl/platform/linux/bsl/cstdlib.hpp

Line 42 in 6509cff

return ::exit(EXIT_FAILURE); // GRCOV_EXCLUDE
is_convertible has to be manually implemented. __is_convertible_to() is not available in GCC.
make_integer_sequence needs a fix

    template<typename T, T N>
    using make_integer_sequence = 
#if __has_builtin(__make_integer_seq)
        __make_integer_seq<integer_sequence, T, N>;
#else
        integer_sequence<T, __integer_pack(N)...>;
#endif

Doxygen/ansi flags is not supported in GCC.
-Weverything is considered a unreliable clang development only flag. See here. I recommend this instead:

set(CLANG_WARNINGS
      -Wall
      -Wextra # reasonable and standard
      -Wshadow # warn the user if a variable declaration shadows one from a parent context
      -Wnon-virtual-dtor # warn the user if a class with virtual functions has a non-virtual destructor. This helps
                         # catch hard to track down memory errors
      -Wold-style-cast # warn for c-style casts
      -Wcast-align # warn for potential performance problem casts
      -Wunused # warn on anything being unused
      -Woverloaded-virtual # warn if you overload (not override) a virtual function
      -Wpedantic # warn if non-standard C++ is used
      -Wconversion # warn on type conversions that may lose data
      -Wsign-conversion # warn on sign conversions
      -Wnull-dereference # warn if a null dereference is detected
      -Wdouble-promotion # warn if float is implicit promoted to double
      -Wformat=2 # warn on security issues around functions that format output (ie printf)
      -Wimplicit-fallthrough # warn on statements that fallthrough without an explicit annotation
  )
  set(GCC_WARNINGS
      ${CLANG_WARNINGS}
      -Wmisleading-indentation # warn if indentation implies blocks where blocks do not exist
      -Wduplicated-cond # warn if if / else chain has duplicated conditions
      -Wduplicated-branches # warn if if / else branches have duplicated code
      -Wlogical-op # warn about logical operations being used where bitwise were probably wanted
      #-Wuseless-cast # warn if you perform a cast to the same type (a lot of casting to same type exists in bsl)
  )

Preliminary test of GCC