c++: Fix pragma suppression of -Wc++20-compat diagnostics [PR106423]

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).

	PR c++/106423

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/cp/ChangeLog:
	* parser.cc (cp_lexer_saving_tokens): Add comment regarding
	diagnostic requirements.

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.

5 files changed, 41 insertions, 2 deletions

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index 9833e50..337a524 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 44e1a60..dfdebd5 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/cp/parser.cc b/gcc/cp/parser.cc
index 33926d2..68fc78e 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -924,7 +924,10 @@ cp_lexer_saving_tokens (const cp_lexer* lexer)
 /* Store the next token from the preprocessor in *TOKEN.  Return true
    if we reach EOF.  If LEXER is NULL, assume we are handling an
    initial #pragma pch_preprocess, and thus want the lexer to return
-   processed strings.  */
+   processed strings.
+
+   Diagnostics issued from this function must have their controlling option (if
+   any) in c.opt annotated as a libcpp option via the CppReason property.  */
 
 static void
 cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token)
diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C b/gcc/testsuite/g++.dg/cpp0x/keywords2.C
new file mode 100644
index 0000000..d67d01e
--- /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 0000000..8714a7b
--- /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;