Improve error message for constexpr constructors of virtual base classes

The changes are for better diagnostic/error-messages. The error message
of Clang, MSVC, and GCC were compared and MSVC gives more detailed
error message so that is used now.

Fixes #64843
Differential Revision: https://reviews.llvm.org/D158540

clang/docs/ReleaseNotes.rst

@@ -216,6 +216,9 @@ Improvements to Clang's diagnostics
 - The fix-it emitted by ``-Wformat`` for scoped enumerations now take the
   enumeration's underlying type into account instead of suggesting a type just
   based on the format string specifier being used.
+- Clang now displays an improved diagnostic and a note when a defaulted special
+  member is marked ``constexpr`` in a class with a virtual base class
+  (`#64843: <https://github.com/llvm/llvm-project/issues/64843>`_).
 
 Bug Fixes in This Version
 -------------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9454,6 +9454,8 @@ def err_defaulted_copy_assign_not_ref : Error<
 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">;

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7819,10 +7819,17 @@ bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
                                   : 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;
   }

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp

@@ -283,3 +283,12 @@ namespace std_example {
     return r;
   }
 }
+
+struct Base {
+ constexpr Base() = default;
+};
+struct Derived : virtual Base { // expected-note 3{{virtual base class declared here}}
+  constexpr Derived() = default; // expected-error {{default constructor cannot be 'constexpr' in a class with virtual base class}}
+  constexpr Derived(const Derived&) = default; // expected-error {{copy constructor cannot be 'constexpr' in a class with virtual base class}}
+  constexpr Derived(Derived&&) = default; // expected-error {{move constructor cannot be 'constexpr' in a class with virtual base class}}
+};