[PATCH] D158540: Improve error message for constexpr constructors of virtual base classes

[Nouman Amir via Phabricator via cfe-commits]

NoumanAmir657 updated this revision to Diff 557504.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D158540/new/

https://reviews.llvm.org/D158540

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Sema/SemaDeclCXX.cpp
  clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp


Index: clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
===================================================================
--- clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
+++ clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
@@ -23,6 +23,15 @@
   NoCopyMove ncm;
 };
 
+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}}
+};
+
 // If a function is explicitly defaulted on its first declaration
 //   -- it is implicitly considered to be constexpr if the implicit declaration
 //      would be
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;
   }
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/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -144,6 +144,8 @@
   tautologies like ``x && !x`` and ``!x || x`` in expressions. This also
   makes ``-Winfinite-recursion`` diagnose more cases.
   (`#56035: <https://github.com/llvm/llvm-project/issues/56035>`_).
+- Clang now displays an improved diagnostic and a note when defaulted special 
+  member is a contexpr in a class with virtual base class
 
 Bug Fixes in This Version
 -------------------------


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