[clang] [clang] Avoid triggering vtable instantiation for C++23 constexpr dtor (PR #102605)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Mariya Podchishchaeva (Fznamznon)

<details>
<summary>Changes</summary>

In C++23 anything can be constexpr, including a dtor of a class whose members and bases don't have constexpr dtors. Avoid early triggering of vtable instantiation int this case.

Fixes https://github.com/llvm/llvm-project/issues/102293

---
Full diff: https://github.com/llvm/llvm-project/pull/102605.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaDeclCXX.cpp (+27-1) 
- (added) clang/test/SemaCXX/gh102293.cpp (+22) 


``````````diff
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index b07e555afcaccf..63d2131acdd650 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7042,12 +7042,38 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
       }
     }
 
+    bool EffectivelyConstexprDestructor = true;
+    // Avoid triggering vtable instantiation due to a dtor that is not
+    // "effectively constexpr" for better compatibility.
+    if (isa<CXXDestructorDecl>(M)) {
+      auto Check = [](QualType T, auto &&Check) -> bool {
+        const CXXRecordDecl *RD =
+            T->getBaseElementTypeUnsafe()->getAsCXXRecordDecl();
+        if (!RD || !RD->isCompleteDefinition())
+          return true;
+
+        if (!RD->hasConstexprDestructor())
+          return false;
+
+        for (const CXXBaseSpecifier &B : RD->bases())
+          if (!Check(B.getType(), Check))
+            return false;
+        for (const FieldDecl *FD : RD->fields())
+          if (!Check(FD->getType(), Check))
+            return false;
+        return true;
+      };
+      EffectivelyConstexprDestructor =
+          Check(QualType(Record->getTypeForDecl(), 0), Check);
+    }
+
     // Define defaulted constexpr virtual functions that override a base class
     // function right away.
     // FIXME: We can defer doing this until the vtable is marked as used.
     if (CSM != CXXSpecialMemberKind::Invalid && !M->isDeleted() &&
         M->isDefaulted() && M->isConstexpr() && M->size_overridden_methods())
-      DefineDefaultedFunction(*this, M, M->getLocation());
+      if (EffectivelyConstexprDestructor)
+        DefineDefaultedFunction(*this, M, M->getLocation());
 
     if (!Incomplete)
       CheckCompletedMemberFunction(M);
diff --git a/clang/test/SemaCXX/gh102293.cpp b/clang/test/SemaCXX/gh102293.cpp
new file mode 100644
index 00000000000000..30629fc03bf6a9
--- /dev/null
+++ b/clang/test/SemaCXX/gh102293.cpp
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+template <typename T> static void destroy() {
+    T t;
+    ++t;
+}
+
+struct Incomplete;
+
+template <typename = int> struct HasD {
+  ~HasD() { destroy<Incomplete*>(); }
+};
+
+struct HasVT {
+  virtual ~HasVT();
+};
+
+struct S : HasVT {
+  HasD<> v;
+};
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/102605