Fix a diagnoses-valid in C++20 with variadic macros

[aaron.ballman]

Summary

C++20 and later allow you to pass no argument for the ... parameter in
a variadic macro, whereas earlier language modes and C disallow it.

We no longer diagnose in C++20 and later modes. This fixes PR51609.

[aaron.ballman]

Committed in af971365a2a8b0d982814c0652bb86844fd19cda

clang/include/clang/Basic/DiagnosticLexKinds.td

 def err_embedded_directive : Error<
   "embedding a #%0 directive within macro arguments is not supported">;
 def ext_embedded_directive : Extension<
   "embedding a directive within macro arguments has undefined behavior">,
   InGroup<DiagGroup<"embedded-directive">>;
 def ext_missing_varargs_arg : Extension<
   "must specify at least one argument for '...' parameter of variadic macro">,
   InGroup<GNUZeroVariadicMacroArguments>;
+def warn_cxx17_compat_missing_varargs_arg : Warning<
+  "passing no argument for the '...' parameter of a variadic macro is "
+  "incompatible with C++ standards before C++20">,
+  InGroup<CXXPre20Compat>, DefaultIgnore;
 def ext_empty_fnmacro_arg : Extension<
   "empty macro arguments are a C99 feature">, InGroup<C99>;
 def warn_cxx98_compat_empty_fnmacro_arg : Warning<
   "empty macro arguments are incompatible with C++98">,
   InGroup<CXX98CompatPedantic>, DefaultIgnore;
 def note_macro_here : Note<"macro %0 defined here">;
 def note_macro_expansion_here : Note<"expansion of macro %0 requested here">;
 

clang/lib/Lex/PPMacroExpansion.cpp

if (NumActuals == 0 && MinArgsExpected == 1) {
                 (NumActuals == 0 && MinArgsExpected == 2))) {// A(x,...) -> A()
       // Varargs where the named vararg parameter is missing: OK as extension.
       //   #define A(x, ...)
       //   A("blah")
       //
       // If the macro contains the comma pasting extension, the diagnostic
       // is suppressed; we know we'll get another diagnostic later.
       if (!MI->hasCommaPasting()) {
-        Diag(Tok, diag::ext_missing_varargs_arg);
+        // C++20 allows this construct, but standards before C++20 and all C
+        // standards do not allow the construct (we allow it as an extension).
+        Diag(Tok, getLangOpts().CPlusPlus20
+                      ? diag::warn_cxx17_compat_missing_varargs_arg
+                      : diag::ext_missing_varargs_arg);
         Diag(MI->getDefinitionLoc(), diag::note_macro_here)
           << MacroName.getIdentifierInfo();
       }
 
       // Remember this occurred, allowing us to elide the comma when used for
       // cases like:
       //   #define A(x, foo...) blah(a, ## foo)
       //   #define B(x, ...) blah(a, ## __VA_ARGS__)

clang/test/Preprocessor/empty_va_arg.cpp

+// RUN: %clang_cc1 -Eonly -std=c++17 -pedantic -verify %s
+// RUN: %clang_cc1 -Eonly -std=c17 -pedantic -verify -x c %s
+// RUN: %clang_cc1 -Eonly -std=c++20 -pedantic -Wpre-c++20-compat -verify=compat %s
+
+#define FOO(x, ...) // expected-note {{macro 'FOO' defined here}} \
+                    // compat-note {{macro 'FOO' defined here}}
+
+int main() {
+  FOO(42) // expected-warning {{must specify at least one argument for '...' parameter of variadic macro}} \
+          // compat-warning {{passing no argument for the '...' parameter of a variadic macro is incompatible with C++ standards before C++20}}
+}
+