[c++20] Implement P0482R6: enable -fchar8_t by default in C++20 mode.

This unfortunately results in a substantial breaking change when
switching to C++20, but it's not yet clear what / how much we should
do about that. We may want to add a compatibility conversion from
u8 string literals to const char*, similar to how C++98 provided a
compatibility conversion from string literals to non-const char*,
but that's not handled by this patch.

The feature can be disabled in C++20 mode with -fno-char8_t.

llvm-svn: 346892

2 files changed, 5 insertions, 5 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 4a3195c69c3..1ea3df4303d 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2475,7 +2475,7 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.ImplicitModules = !Args.hasArg(OPT_fno_implicit_modules);
   Opts.CharIsSigned = Opts.OpenCL || !Args.hasArg(OPT_fno_signed_char);
   Opts.WChar = Opts.CPlusPlus && !Args.hasArg(OPT_fno_wchar);
-  Opts.Char8 = Args.hasArg(OPT_fchar8__t);
+  Opts.Char8 = Args.hasFlag(OPT_fchar8__t, OPT_fno_char8__t, Opts.CPlusPlus2a);
   if (const Arg *A = Args.getLastArg(OPT_fwchar_type_EQ)) {
     Opts.WCharSize = llvm::StringSwitch<unsigned>(A->getValue())
                          .Case("char", 1)
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index db707f46b84..b8d808f2d38 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -558,15 +558,15 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   if (LangOpts.RelaxedTemplateTemplateArgs)
     Builder.defineMacro("__cpp_template_template_args", "201611L");
 
+  // C++20 features.
+  if (LangOpts.Char8)
+    Builder.defineMacro("__cpp_char8_t", "201811L");
+
   // TS features.
   if (LangOpts.ConceptsTS)
     Builder.defineMacro("__cpp_experimental_concepts", "1L");
   if (LangOpts.CoroutinesTS)
     Builder.defineMacro("__cpp_coroutines", "201703L");
-
-  // Potential future breaking changes.
-  if (LangOpts.Char8)
-    Builder.defineMacro("__cpp_char8_t", "201803L");
 }
 
 static void InitializePredefinedMacros(const TargetInfo &TI,