Index: clang/include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticLexKinds.td
+++ clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -412,6 +412,10 @@
 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<
Index: clang/lib/Lex/PPMacroExpansion.cpp
===================================================================
--- clang/lib/Lex/PPMacroExpansion.cpp
+++ clang/lib/Lex/PPMacroExpansion.cpp
@@ -988,7 +988,11 @@
       // 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();
       }
Index: clang/test/Preprocessor/empty_va_arg.cpp
===================================================================
--- /dev/null
+++ clang/test/Preprocessor/empty_va_arg.cpp
@@ -0,0 +1,12 @@
+// 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}}
+}
+