C++17 std::byte detection fails on ubuntu+clang-6.0

[gnbond]

Describe the bug
Clang6 and clang8 in c++17 mode cannot compile catch unit tests

Expected behavior
C++17 should be supported

Reproduction steps
Ubuntu 16, clang++6.0 (and clang++-8) installed with "apt install clang6.0"
Simple null unit test:

#include "catch.hpp"
TEST_CASE("simple", "[simple]") {
}

$ clang++-6.0 -std=c++17 -c cb.cpp 
In file included from cb.cpp:1:
./catch.hpp:1735:29: error: no member named 'byte' in namespace 'std'
    struct StringMaker<std::byte> {
                       ~~~~~^
1 error generated.

Platform information:

• OS: ubuntu 16

• Compiler+version:
  clang version 6.0.0-1ubuntu2~16.04.1 (tags/RELEASE_600/final)
  Target: x86_64-pc-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin

• Catch version: v2.13.0 downloaded in the single file version

Additional context
Also happens with clang-8 package.
Compiles fine in clang-6 or clang-8 in c+11 or c++14 mode.

This may be a packaging error in ubuntu, or it might be I have not installed some vital piece, or need to configure clang to use a better standard library or something.

[arobenko]

@gnbond
I had similar problem and worked around it by adding the CATCH_CONFIG_NO_CPP17_BYTE definition:

    target_compile_options(my_binary PRIVATE
      $<$<CXX_COMPILER_ID:Clang>:-DCATCH_CONFIG_NO_CPP17_BYTE=1>
    )

[gnbond]

After a bunch more experimentation, this appears to be a ubuntu 16-specific issue, clang packages on ubuntu-18 (and FreeBSD 11) work fine. I think the issue is the clang packages are still using OS-default standard library, which is g++5 on ubuntu-16, and g++5 version of <cstddef> does not have std::byte.

The below patch seems to fix it, and seems in the spirit of the feature test macros , but I do not know if there are any adverse consequences of #including in this circumstance.

(line numbers are the single-file version 2.13.0)

 --- a/catch.hpp
+++ b/catch.hpp
@@ -330,7 +330,10 @@ namespace Catch {
 
   // Check if byte is available and usable
   #  if __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)
-  #    define CATCH_INTERNAL_CONFIG_CPP17_BYTE
+  #    include <cstddef>
+  #    if __cpp_lib_byte
+  #      define CATCH_INTERNAL_CONFIG_CPP17_BYTE
+  #    endif
   #  endif // __has_include(<cstddef>) && defined(CATCH_CPP17_OR_GREATER)
 
   // Check if variant is available and usable

[horenmar]

Yeah, some feature checks still use the old and wrong way of doing them -- it is a legacy of most feature checks being for C++11/14, which did not have feature macros, so checking for C++ standard and then hoping for the best was a reasonable option.

If you open up a PR, I will merge it, otherwise it will have to wait until I get around to working on v2 branch again.