Fix libc++'s lit configuration language dialect inference for GCC 5 and GCC 6

[• wash]

Summary

libc++'s lit configuration infers the C++ language dialect when it is not provided by checking which -std= flags that a compiler supports. GCC 5 and GCC 6 have a -std=c++17 flag, however, they do not have full C++17 support. The lit configuration has hardcoded logic that removes -std=c++1z as an option to test for GCC < 7, but not -std=c++17.

This leads to a bunch of failures when running libc++ tests with GCC 5 or GCC 6.

This patch adds -std=c++17 to the list of flags that are discarded for GCC < 7 by lit's language dialect inference.

[ldionne]

I'm okay with this (I don't care much about GCC), but I want to point out you're basically reducing the usefulness of the test suite on GCC 5/6 by just wholesale removing all C++17 tests, when GCC5/6 can actually run a large subset of the tests.

I think this is still better than adding a bunch of complexity to disable tests in a finer-grained manner.

Do you have commit access?

[• wash]

In D62874#1529811, @ldionne wrote:

I'm okay with this (I don't care much about GCC), but I want to point out you're basically reducing the usefulness of the test suite on GCC 5/6 by just wholesale removing all C++17 tests, when GCC5/6 can actually run a large subset of the tests.

I see your point, but I think it was always the intention to disable these tests universally for GCC < 7 (thus the removal of -std=c++1z). Also, we do the same thing for C++14 tests for GCC5 - none of them run for GCC < 6.

The C++17 language support chart for GCC is pretty sparse until GCC 7.

I thought about going through the failures individually, but there's about 150 with GCC 5.

I think this is still better than adding a bunch of complexity to disable tests in a finer-grained manner.

Agreed.

Do you have commit access?

Probably not.

[ldionne]

Should I commit this? @wash

utils/libcxx/test/config.py

def configure_default_compile_flags(self):
             # Choose the newest possible language dialect if none is given.
             possible_stds = ['c++2a', 'c++17', 'c++1z', 'c++14', 'c++11',
                              'c++03']
             if self.cxx.type == 'gcc':
                 maj_v, _, _ = self.cxx.version
                 maj_v = int(maj_v)
                 if maj_v < 7:
                     possible_stds.remove('c++1z')
+                    possible_stds.remove('c++17')
                 # FIXME: How many C++14 tests actually fail under GCC 5 and 6?
                 # Should we XFAIL them individually instead?
                 if maj_v <= 6:
                     possible_stds.remove('c++14')
             for s in possible_stds:
                 if self.cxx.hasCompileFlag('-std=%s' % s):
                     std = s
                     self.lit_config.note(