preprocessor: Implement C++23 P2437R1 - Support for #warning [PR106646]

On Thu, Aug 18, 2022 at 11:02:44PM +0000, Joseph Myers wrote:
> ISO C2x standardizes the existing #warning extension.  Arrange
> accordingly for it not to be diagnosed with -std=c2x -pedantic, but to
> be diagnosed with -Wc11-c2x-compat.

And here is the corresponding C++ version.
Don't pedwarn about this for C++23/GNU++23 and tweak the diagnostics
for C++ otherwise, + testsuite coverage.
The diagnostic wording is similar e.g. to the #elifdef diagnostics.

2022-08-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106646
	* init.cc: Implement C++23 P2437R1 - Support for #warning.
	(lang_defaults): Set warning_directive for GNUCXX23 and CXX23.
	* directives.cc (directive_diagnostics): Use different wording of
	#warning pedwarn for C++.

	* g++.dg/cpp/warning-1.C: New test.
	* g++.dg/cpp/warning-2.C: New test.
	* g++.dg/cpp/warning-3.C: New test.

5 files changed, 28 insertions, 4 deletions

diff --git a/gcc/testsuite/g++.dg/cpp/warning-1.C b/gcc/testsuite/g++.dg/cpp/warning-1.C
new file mode 100644
index 0000000..2d857cf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp/warning-1.C
@@ -0,0 +1,6 @@
+// P2437R1 - Support for #warning
+// { dg-do preprocess }
+// { dg-options "-pedantic-errors" }
+
+#warning example text /* { dg-warning "example text" } */
+// { dg-error "#warning before C\\\+\\\+23 is a GCC extension" "pedantic" { target c++20_down } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp/warning-2.C b/gcc/testsuite/g++.dg/cpp/warning-2.C
new file mode 100644
index 0000000..d6d5d9f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp/warning-2.C
@@ -0,0 +1,6 @@
+// P2437R1 - Support for #warning
+// { dg-do preprocess }
+// { dg-options "-pedantic" }
+
+#warning example text /* { dg-warning "example text" } */
+// { dg-warning "#warning before C\\\+\\\+23 is a GCC extension" "pedantic" { target c++20_down } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp/warning-3.C b/gcc/testsuite/g++.dg/cpp/warning-3.C
new file mode 100644
index 0000000..1595b4d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp/warning-3.C
@@ -0,0 +1,6 @@
+// P2437R1 - Support for #warning
+// { dg-do preprocess }
+// { dg-options "" }
+
+#warning example text /* { dg-warning "example text" } */
+// { dg-bogus "#warning before C\\\+\\\+23 is a GCC extension" "" { target *-*-* } .-1 }
diff --git a/libcpp/directives.cc b/libcpp/directives.cc
index 802bd8c..918752f 100644
--- a/libcpp/directives.cc
+++ b/libcpp/directives.cc
@@ -388,8 +388,14 @@ directive_diagnostics (cpp_reader *pfile, const directive *dir, int indented)
       else if (dir == &dtable[T_WARNING])
 	{
 	  if (CPP_PEDANTIC (pfile) && !CPP_OPTION (pfile, warning_directive))
-	    cpp_error (pfile, CPP_DL_PEDWARN,
-		       "#%s before C2X is a GCC extension", dir->name);
+	    {
+	      if (CPP_OPTION (pfile, cplusplus))
+		cpp_error (pfile, CPP_DL_PEDWARN,
+			   "#%s before C++23 is a GCC extension", dir->name);
+	      else
+		cpp_error (pfile, CPP_DL_PEDWARN,
+			   "#%s before C2X is a GCC extension", dir->name);
+	    }
 	  else if (CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) > 0)
 	    cpp_warning (pfile, CPP_W_C11_C2X_COMPAT,
 			 "#%s before C2X is a GCC extension", dir->name);
diff --git a/libcpp/init.cc b/libcpp/init.cc
index b184e1f..39e7e75 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -123,8 +123,8 @@ static const struct lang_flags lang_defaults[] =
   /* CXX17    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,     1,     0,   1,      0,   1,     0,   0,   0,      0,      0 },
   /* GNUCXX20 */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   0,   0,      0,      0 },
   /* CXX20    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   0,   0,      0,      0 },
-  /* GNUCXX23 */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      0,      1 },
-  /* CXX23    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      0,      1 },
+  /* GNUCXX23 */  { 1,  1,  1,  1,  1,  0,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1 },
+  /* CXX23    */  { 1,  1,  1,  1,  1,  1,  1,   1,   1,   1,    1,     1,     0,   1,      1,   1,     0,   1,   1,      1,      1 },
   /* ASM      */  { 0,  0,  1,  0,  0,  0,  0,   0,   0,   0,    0,     0,     0,   0,      0,   0,     0,   0,   0,      0,      0 }
 };