[PATCH 1/1] c++/106423: Fix pragma suppression of -Wc++20-compat diagnostics.

[Tom Honermann <tom@honermann.net>]

Gcc's '#pragma GCC diagnostic' directives are processed in "early mode"
(see handle_pragma_diagnostic_early) for the C++ frontend and, as such,
require that the target diagnostic option be enabled for the preprocessor
(see c_option_is_from_cpp_diagnostics).  This change modifies the
-Wc++20-compat option definition to register it as a preprocessor option
so that its associated diagnostics can be suppressed.  The changes also
implicitly disable the option in C++20 and later modes.  These changes
are consistent with the definition of the -Wc++11-compat option.

This support is motivated by the need to suppress the following diagnostic
otherwise issued in C++17 and earlier modes due to the char8_t typedef
present in the uchar.h header file in glibc 2.36.
  warning: identifier ‘char8_t’ is a keyword in C++20 [-Wc++20-compat]

Tests are added to validate suppression of both -Wc++11-compat and
-Wc++20-compat related diagnostics (fixes were only needed for the C++20
case).

Fixes https://gcc.gnu.org/PR106423.

gcc/c-family/ChangeLog:
	* c-opts.cc (c_common_post_options): Disable -Wc++20-compat diagnostics
	in C++20 and later.
	* c.opt (Wc++20-compat): Enable hooks for the preprocessor.

gcc/testsuite/ChangeLog:
	* g++.dg/cpp0x/keywords2.C: New test.
	* g++.dg/cpp2a/keywords2.C: New test.

libcpp/ChangeLog:
	* include/cpplib.h (cpp_warning_reason): Add CPP_W_CXX20_COMPAT.
	* init.cc (cpp_create_reader): Add cpp_warn_cxx20_compat.
---
 gcc/c-family/c-opts.cc                 |  7 +++++++
 gcc/c-family/c.opt                     |  2 +-
 gcc/testsuite/g++.dg/cpp0x/keywords2.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/keywords2.C | 13 +++++++++++++
 libcpp/include/cpplib.h                |  4 ++++
 libcpp/init.cc                         |  1 +
 6 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/keywords2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/keywords2.C

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index b9f01a65ed7..1ea37ba9742 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -1046,6 +1046,13 @@ c_common_post_options (const char **pfilename)
   else if (warn_narrowing == -1)
     warn_narrowing = 0;
 
+  if (cxx_dialect >= cxx20)
+    {
+      /* Don't warn about C++20 compatibility changes in C++20 or later.  */
+      warn_cxx20_compat = 0;
+      cpp_opts->cpp_warn_cxx20_compat = 0;
+    }
+
   /* C++17 has stricter evaluation order requirements; let's use some of them
      for earlier C++ as well, so chaining works as expected.  */
   if (c_dialect_cxx ()
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 44e1a60ce24..dfdebd596ef 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -455,7 +455,7 @@ Wc++2a-compat
 C++ ObjC++ Warning Alias(Wc++20-compat) Undocumented
 
 Wc++20-compat
-C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall)
+C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall) Init(0) CPP(cpp_warn_cxx20_compat) CppReason(CPP_W_CXX20_COMPAT)
 Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020.
 
 Wc++11-extensions
diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
new file mode 100644
index 00000000000..d67d01e31ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++98_only } }
+// { dg-options "-Wc++11-compat" }
+
+// Validate suppression of -Wc++11-compat diagnostics.
+#pragma GCC diagnostic ignored "-Wc++11-compat"
+int alignof;
+int alignas;
+int constexpr;
+int decltype;
+int noexcept;
+int nullptr;
+int static_assert;
+int thread_local;
+int _Alignas;
+int _Alignof;
+int _Thread_local;
diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords2.C b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
new file mode 100644
index 00000000000..8714a7b26b7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/keywords2.C
@@ -0,0 +1,13 @@
+// { dg-do compile { target c++17_down } }
+// { dg-options "-Wc++20-compat" }
+
+// Validate suppression of -Wc++20-compat diagnostics.
+#pragma GCC diagnostic ignored "-Wc++20-compat"
+int constinit;
+int consteval;
+int requires;
+int concept;
+int co_await;
+int co_yield;
+int co_return;
+int char8_t;
diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index 3eba6f74b57..9d90c18e4f2 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -547,6 +547,9 @@ struct cpp_options
   /* True if warn about differences between C++98 and C++11.  */
   bool cpp_warn_cxx11_compat;
 
+  /* True if warn about differences between C++17 and C++20.  */
+  bool cpp_warn_cxx20_compat;
+
   /* Nonzero if bidirectional control characters checking is on.  See enum
      cpp_bidirectional_level.  */
   unsigned char cpp_warn_bidirectional;
@@ -655,6 +658,7 @@ enum cpp_warning_reason {
   CPP_W_C90_C99_COMPAT,
   CPP_W_C11_C2X_COMPAT,
   CPP_W_CXX11_COMPAT,
+  CPP_W_CXX20_COMPAT,
   CPP_W_EXPANSION_TO_DEFINED,
   CPP_W_BIDIRECTIONAL
 };
diff --git a/libcpp/init.cc b/libcpp/init.cc
index f4ab83d2145..cca3c1dc1e7 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -202,6 +202,7 @@ cpp_create_reader (enum c_lang lang, cpp_hash_table *table,
   CPP_OPTION (pfile, cpp_warn_c90_c99_compat) = -1;
   CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) = -1;
   CPP_OPTION (pfile, cpp_warn_cxx11_compat) = 0;
+  CPP_OPTION (pfile, cpp_warn_cxx20_compat) = 0;
   CPP_OPTION (pfile, cpp_warn_deprecated) = 1;
   CPP_OPTION (pfile, cpp_warn_long_long) = 0;
   CPP_OPTION (pfile, dollars_in_ident) = 1;
-- 
2.32.0