[Clang] Adjust extension warnings for #warning

[cor3ntin]

Summary

The #warning directive is standard in C++2b and C2x,
this adjusts the pedantic and extensions warning accordingly.

[cor3ntin]

Formatting

[cor3ntin]

Update release notes.

[aaron.ballman]

LGTM!

clang/include/clang/Basic/DiagnosticLexKinds.td

393–398

<Not Your Problem>Someday, it would be nice if we made the tablegen for diagnostics a wee bit smarter so that we don't need to use two separate warning declarations for this situation. In fact, it'd be nice if tablegen could just automate this pattern given how often we use it and will be using it in the future.</Not Your Problem>

clang/lib/Lex/PPDirectives.cpp

1264–1274

Similarly (and also not your problem), it'd be nice to have a helper function so that we don't need this complicated of a dance to diagnose the situation.

[cor3ntin]

clang/include/clang/Basic/DiagnosticLexKinds.td

393–398

That would be great indeed!

clang/lib/Lex/PPDirectives.cpp

1264–1274

Agreed!

clang/docs/ReleaseNotes.rst

- ``-Wenum-conversion`` now warns on converting a signed enum of one type to an
   ``-Wsign-conversion``.
 - Added the ``-Wunreachable-code-generic-assoc`` diagnostic flag (grouped under
   the ``-Wunreachable-code`` flag) which is enabled by default and warns the
   user about ``_Generic`` selection associations which are unreachable because
   the type specified is an array type or a qualified type.
 - Added the ``-Wgnu-line-marker`` diagnostic flag (grouped under the ``-Wgnu``
   flag) which is a portability warning about use of GNU linemarker preprocessor
   directives. Fixes `Issue 55067 <https://github.com/llvm/llvm-project/issues/55067>`_.
-- Using ``#elifdef`` and ``#elifndef`` that are incompatible with C/C++
+- Using ``#warning``, ``#elifdef`` and ``#elifndef`` that are incompatible with C/C++
   standards before C2x/C++2b are now warned via ``-pedantic``. Additionally,
   on such language mode, ``-Wpre-c2x-compat`` and ``-Wpre-c++2b-compat``
   diagnostic flags report a compatibility issue.
   Fixes `Issue 55306 <https://github.com/llvm/llvm-project/issues/55306>`_.
 - Clang now checks for stack resource exhaustion when recursively parsing
   declarators in order to give a diagnostic before we run out of stack space.
   This fixes `Issue 51642 <https://github.com/llvm/llvm-project/issues/51642>`_.
 - Unknown preprocessor directives in a skipped conditional block are now given

clang/include/clang/Basic/DiagnosticLexKinds.td

def err_pp_import_directive_ms : Error<
   "#import of type library is an unsupported Microsoft feature">;
 def ext_pp_include_search_ms : ExtWarn<
   "#include resolved using non-portable Microsoft search rules as: %0">,
   InGroup<MicrosoftInclude>;
 
 def ext_pp_ident_directive : Extension<"#ident is a language extension">;
 def ext_pp_include_next_directive : Extension<
   "#include_next is a language extension">, InGroup<GNUIncludeNext>;
-def ext_pp_warning_directive : Extension<"#warning is a language extension">;
+
+def ext_pp_warning_directive : Extension<
+  "#warning is a %select{C2x|C++2b}0 extension">;
+def warn_cxx2b_compat_warning_directive : Warning<
+  "#warning is incompatible with C++ standards before C++2b">,
+  InGroup<CXXPre2bCompat>, DefaultIgnore;
+def warn_c2x_compat_warning_directive : Warning<
+  "#warning is incompatible with C standards before C2x">,
+  InGroup<CPre2xCompat>, DefaultIgnore;
 
 def ext_pp_extra_tokens_at_eol : ExtWarn<
   "extra tokens at end of #%0 directive">, InGroup<ExtraTokens>;
 
 def ext_pp_comma_expr : Extension<"comma operator in operand of #if">;
 def ext_pp_bad_vaargs_use : Extension<
   "__VA_ARGS__ can only appear in the expansion of a C99 variadic macro">;
 

clang/lib/Lex/PPDirectives.cpp

default:
 
     // GNU Extensions.
     case tok::pp_import:
       return HandleImportDirective(SavedHash.getLocation(), Result);
     case tok::pp_include_next:
       return HandleIncludeNextDirective(SavedHash.getLocation(), Result);
 
     case tok::pp_warning:
-      Diag(Result, diag::ext_pp_warning_directive);
+      if (LangOpts.CPlusPlus)
+        Diag(Result, LangOpts.CPlusPlus2b
+                         ? diag::warn_cxx2b_compat_warning_directive
+                         : diag::ext_pp_warning_directive)
+            << /*C++2b*/ 1;
+      else
+        Diag(Result, LangOpts.C2x ? diag::warn_c2x_compat_warning_directive
+                                  : diag::ext_pp_warning_directive)
+            << /*C2x*/ 0;
+
       return HandleUserDiagnosticDirective(Result, true);
     case tok::pp_ident:
       return HandleIdentSCCSDirective(Result);
     case tok::pp_sccs:
       return HandleIdentSCCSDirective(Result);
     case tok::pp_assert:
       //isExtension = true;  // FIXME: implement #assert
       break;
     case tok::pp_unassert:

clang/test/Preprocessor/ext-pp-directive.c

 #endif
 // For C
 // pre-c2x-pedantic-warning@-3 {{use of a '#elifndef' directive is a C2x extension}}
 // pre-c2x-compat-warning@-4 {{use of a '#elifndef' directive is incompatible with C standards before C2x}}
 
 // For C++
 // pre-cpp2b-pedantic-warning@-7 {{use of a '#elifndef' directive is a C++2b extension}}
 // pre-cpp2b-compat-warning@-8 {{use of a '#elifndef' directive is incompatible with C++ standards before C++2b}}
+
+#warning foo
+// For C
+// pre-c2x-pedantic-warning@-2 {{#warning is a C2x extension}}
+// pre-c2x-pedantic-warning@-3 {{foo}}
+// pre-c2x-compat-warning@-4 {{#warning is incompatible with C standards before C2x}}
+// pre-c2x-compat-warning@-5 {{foo}}
+
+// For C++
+// pre-cpp2b-pedantic-warning@-8 {{#warning is a C++2b extension}}
+// pre-cpp2b-pedantic-warning@-9 {{foo}}
+// pre-cpp2b-compat-warning@-10 {{#warning is incompatible with C++ standards before C++2b}}
+// pre-cpp2b-compat-warning@-11 {{foo}}