[clang] [Clang][Sema] Fix err_constexpr_virtual_base diagnostic so that it only diagnoses on constructors and destructors (PR #163690)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik created https://github.com/llvm/llvm-project/pull/163690

When this diagnostic was implemented it fired for all non-static data members. I added checking so that it only fires for constructors and destructors before C++26 (which now allows this). Some special care was required for the destructor case since constexpr constuctors were not allowed in C++11. Modified existing test to cover the new conditions.

Also filed bug report on bad diagnostic found while writing the tests: https://github.com/llvm/llvm-project/issues/163683

Fixes: https://github.com/llvm/llvm-project/issues/97266

>From f6bded3e92814b7ec5f2c3427e9c61b3fa0775f2 Mon Sep 17 00:00:00 2001
From: Shafik Yaghmour <shafik.yaghmour at intel.com>
Date: Wed, 15 Oct 2025 20:09:36 -0700
Subject: [PATCH] [Clang][Sema] Fix err_constexpr_virtual_base diagnostic so
 that it only diagnoses on constructors and destructors

When this diagnostic was implemented it fired for all non-static data members.
I added checking so that it only fires for contructors and destructors before
C++26 (which now allows this). Some special care was required for the destructor
case since constexpr constuctors were not allowed in C++11. Modified existing
test to cover the new conditions.

Fixes: https://github.com/llvm/llvm-project/issues/97266
---
 clang/include/clang/Basic/DiagnosticSemaKinds.td     |  2 +-
 clang/lib/Sema/SemaDeclCXX.cpp                       | 10 ++++++----
 clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp | 10 ++++++++--
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 40bc7b9a4e45e..0eae2b76ff9b9 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3026,7 +3026,7 @@ def warn_cxx17_compat_constexpr_virtual : Warning<
   "virtual constexpr functions are incompatible with "
   "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 def err_constexpr_virtual_base : Error<
-  "constexpr %select{member function|constructor}0 not allowed in "
+  "constexpr %select{destructor|constructor}0 not allowed in "
   "%select{struct|interface|class}1 with virtual base "
   "%plural{1:class|:classes}2">;
 def note_non_literal_incomplete : Note<
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index d41ab126c426f..da94b0070dfcb 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -1918,8 +1918,10 @@ static bool CheckConstexprMissingReturn(Sema &SemaRef, const FunctionDecl *Dcl);
 
 bool Sema::CheckConstexprFunctionDefinition(const FunctionDecl *NewFD,
                                             CheckConstexprKind Kind) {
-  const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewFD);
-  if (MD && MD->isInstance()) {
+  if ((!getLangOpts().CPlusPlus26 && isa<CXXConstructorDecl>(NewFD)) ||
+      ((getLangOpts().CPlusPlus20 && !getLangOpts().CPlusPlus26) &&
+       isa<CXXDestructorDecl>(NewFD))) {
+    const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewFD);
     // C++11 [dcl.constexpr]p4:
     //  The definition of a constexpr constructor shall satisfy the following
     //  constraints:
@@ -1933,8 +1935,8 @@ bool Sema::CheckConstexprFunctionDefinition(const FunctionDecl *NewFD,
         return false;
 
       Diag(NewFD->getLocation(), diag::err_constexpr_virtual_base)
-        << isa<CXXConstructorDecl>(NewFD)
-        << getRecordDiagFromTagKind(RD->getTagKind()) << RD->getNumVBases();
+          << isa<CXXConstructorDecl>(NewFD)
+          << getRecordDiagFromTagKind(RD->getTagKind()) << RD->getNumVBases();
       for (const auto &I : RD->vbases())
         Diag(I.getBeginLoc(), diag::note_constexpr_virtual_base_here)
             << I.getSourceRange();
diff --git a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
index 51990ee4341d2..5a06d686b4c0f 100644
--- a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
+++ b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
@@ -84,10 +84,16 @@ struct T3 {
 struct U {
   constexpr U SelfReturn() const;
   constexpr int SelfParam(U) const;
+  constexpr ~U();  // beforecxx20-error {{destructor cannot be declared constexpr}}
 };
 
-struct V : virtual U { // expected-note {{here}}
-  constexpr int F() const { return 0; } // expected-error {{constexpr member function not allowed in struct with virtual base class}}
+struct V : virtual U { // expected-note {{here}} //aftercxx20-note {{here}}
+  constexpr V() {}  // expected-error {{constexpr constructor not allowed in struct with virtual base class}}
+  constexpr ~V() {} // aftercxx20-error {{constexpr destructor not allowed in struct with virtual base class}}
+                    // beforecxx20-error at -1 {{destructor cannot be declared constexpr}}
+                    // beforecxx14-error at -2 {{constexpr function's return type 'void' is not a literal type}}
+		    // ^ FIXME this is just a bad diagnostic
+  constexpr int f() const { return 0; }
 };
 
 //  or a compound-statememt that contains only [CXX11]