[libc++] Translate the std Lit parameter to the DSL

libcxx/utils/libcxx/test/config.py

@@ -344,38 +344,6 @@ class Configuration(object):
             self.cxx.compile_flags += shlex.split(additional_flags)
 
     def configure_default_compile_flags(self):
-        # Try and get the std version from the command line. Fall back to
-        # default given in lit.site.cfg is not present. If default is not
-        # present then force c++11.
-        std = self.get_lit_conf('std')
-        if not std:
-            # 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(
-                        'inferred language dialect as: %s' % std)
-                    break
-            if not std:
-                self.lit_config.fatal(
-                    'Failed to infer a supported language dialect from one of %r'
-                    % possible_stds)
-        self.cxx.compile_flags += ['-std={0}'.format(std)]
-        std_feature = std.replace('gnu++', 'c++')
-        std_feature = std.replace('1z', '17')
-        self.config.available_features.add(std_feature)
         # Configure include paths
         self.configure_compile_flags_header_includes()
         self.target_info.add_cxx_compile_flags(self.cxx.compile_flags)

libcxx/utils/libcxx/test/params.py

@@ -8,7 +8,17 @@
 
 from libcxx.test.dsl import *
 
+_allStandards = ['c++98', 'c++03', 'c++11', 'c++14', 'c++17', 'c++2a']
+
 parameters = [
+  # Core parameters of the test suite
+  Parameter(name='std', choices=_allStandards, type=str,
+            help="The version of the standard to compile the test suite with.",
+            default=lambda cfg: next(s for s in reversed(_allStandards) if hasCompileFlag(cfg, '-std='+s)),
+            feature=lambda std:
+              Feature(name=std, compileFlag='-std={}'.format(std),
+                      when=lambda cfg: hasCompileFlag(cfg, '-std={}'.format(std)))),
+
   Parameter(name='enable_exceptions', choices=[True, False], type=bool, default=True,
             help="Whether to enable exceptions when compiling the test suite.",
             feature=lambda exceptions: None if exceptions else