Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9395,6 +9395,8 @@
 def err_incorrect_defaulted_constexpr : Error<
   "defaulted definition of %sub{select_special_member_kind}0 "
   "is not constexpr">;
+def err_incorrect_defaulted_constexpr_with_vb: Error<
+  "%sub{select_special_member_kind}0 cannot be 'constexpr' in a class with virtual base class">;
 def err_incorrect_defaulted_consteval : Error<
   "defaulted declaration of %sub{select_special_member_kind}0 "
   "cannot be consteval because implicit definition is not constexpr">;
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -7800,10 +7800,17 @@
                                   : isa<CXXConstructorDecl>(MD))) &&
       MD->isConstexpr() && !Constexpr &&
       MD->getTemplatedKind() == FunctionDecl::TK_NonTemplate) {
-    Diag(MD->getBeginLoc(), MD->isConsteval()
-                                ? diag::err_incorrect_defaulted_consteval
-                                : diag::err_incorrect_defaulted_constexpr)
-        << CSM;
+    if (!MD->isConsteval() && RD->getNumVBases()) {
+      Diag(MD->getBeginLoc(), diag::err_incorrect_defaulted_constexpr_with_vb)
+          << CSM;
+      for (const auto &I : RD->vbases())
+        Diag(I.getBeginLoc(), diag::note_constexpr_virtual_base_here);
+    } else {
+      Diag(MD->getBeginLoc(), MD->isConsteval()
+                                  ? diag::err_incorrect_defaulted_consteval
+                                  : diag::err_incorrect_defaulted_constexpr)
+          << CSM;
+    }
     // FIXME: Explain why the special member can't be constexpr.
     HadError = true;
   }