[Clang] Fix constexpr-ness on implicitly deleted destructors (#116359)

In C++20, a defaulted but implicitly deleted destructor is constexpr if
and only if the class has no virtual base class. This hasn't been
changed in C++23 by P2448R2.

Constexpr-ness on a deleted destructor affects almost nothing. The
`__is_literal` intrinsic is related, while the corresponding
`std::is_literal_type(_v)` utility has been removed in C++20. A recently
added example in `test/AST/ByteCode/cxx23.cpp` will become valid, and
the example is already accepted by GCC.

Clang currently behaves correctly in C++23 mode, because the
constexpr-ness on defaulted destructor is relaxed by P2448R2. But we
should make similar relaxation for an implicitly deleted destructor.

Fixes #85550.

Author: frederick-vs-ja

clang/docs/ReleaseNotes.rst

@@ -158,6 +158,35 @@ C++ Specific Potentially Breaking Changes
 
   Previously, this code was erroneously accepted.
 
+- Clang will now consider the implicitly deleted destructor of a union or
+  a non-union class without virtual base class to be ``constexpr`` in C++20
+  mode (Clang 19 only did so in C++23 mode but the standard specification for
+  this changed in C++20). (#GH85550)
+
+  .. code-block:: c++
+
+    struct NonLiteral {
+      NonLiteral() {}
+      ~NonLiteral() {}
+    };
+
+    template <class T>
+    struct Opt {
+      union {
+        char c;
+        T data;
+      };
+      bool engaged = false;
+
+      constexpr Opt() {}
+      constexpr ~Opt() {
+        if (engaged)
+          data.~T();
+      }
+    };
+
+    // Previously only accepted in C++23 and later, now also accepted in C++20.
+    consteval void foo() { Opt<NonLiteral>{}; }
 
 ABI Changes in This Version
 ---------------------------

clang/include/clang/AST/DeclCXX.h

@@ -890,6 +890,13 @@ class CXXRecordDecl : public RecordDecl {
             needsOverloadResolutionForDestructor()) &&
            "destructor should not be deleted");
     data().DefaultedDestructorIsDeleted = true;
+    // C++23 [dcl.constexpr]p3.2:
+    //   if the function is a constructor or destructor, its class does not have
+    //   any virtual base classes.
+    // C++20 [dcl.constexpr]p5:
+    //   The definition of a constexpr destructor whose function-body is
+    //   not = delete shall additionally satisfy...
+    data().DefaultedDestructorIsConstexpr = data().NumVBases == 0;
   }
 
   /// Determine whether this class should get an implicit move

clang/lib/AST/DeclCXX.cpp

@@ -546,9 +546,9 @@ void CXXRecordDecl::addedClassSubobject(CXXRecordDecl *Subobj) {
     data().NeedOverloadResolutionForDestructor = true;
   }
 
-  // C++2a [dcl.constexpr]p4:
-  //   The definition of a constexpr destructor [shall] satisfy the
-  //   following requirement:
+  // C++20 [dcl.constexpr]p5:
+  //  The definition of a constexpr destructor whose function-body is not
+  //  = delete shall additionally satisfy the following requirement:
   //   -- for every subobject of class type or (possibly multi-dimensional)
   //      array thereof, that class type shall have a constexpr destructor
   if (!Subobj->hasConstexprDestructor())
@@ -1213,8 +1213,13 @@ void CXXRecordDecl::addedMember(Decl *D) {
             data().DefaultedCopyAssignmentIsDeleted = true;
           if (FieldRec->hasNonTrivialMoveAssignment())
             data().DefaultedMoveAssignmentIsDeleted = true;
-          if (FieldRec->hasNonTrivialDestructor())
+          if (FieldRec->hasNonTrivialDestructor()) {
             data().DefaultedDestructorIsDeleted = true;
+            // C++20 [dcl.constexpr]p5:
+            //   The definition of a constexpr destructor whose function-body is
+            //   not = delete shall additionally satisfy...
+            data().DefaultedDestructorIsConstexpr = true;
+          }
         }
 
         // For an anonymous union member, our overload resolution will perform

clang/test/AST/ByteCode/cxx23.cpp

@@ -263,7 +263,7 @@ namespace AnonUnionDtor {
   template <class T>
   struct opt
   {
-    union { // all20-note {{is not literal}}
+    union {
       char c;
       T data;
     };
@@ -279,7 +279,7 @@ namespace AnonUnionDtor {
   };
 
   consteval void foo() {
-    opt<A> a; // all20-error {{variable of non-literal type}}
+    opt<A> a;
   }
 
   void bar() { foo(); }

clang/test/SemaCXX/literal-type.cpp

@@ -20,6 +20,7 @@ static_assert(__is_literal(VectorExt), "fail");
 //    [...]
 //    -- a class type that has all of the following properties:
 //        -- it has a trivial destructor
+//           [P0784R7 changed the condition to "constexpr destructor" in C++20]
 //        -- every constructor call and full-expression in the
 //           brace-or-equal-initializers for non-static data members (if an) is
 //           a constant expression,
@@ -108,3 +109,70 @@ void test() {
 
 }
 #endif
+
+#if __cplusplus >= 201103L
+namespace GH85550 {
+struct HasDefaultCtorAndNonConstexprDtor {
+  constexpr HasDefaultCtorAndNonConstexprDtor() = default;
+  ~HasDefaultCtorAndNonConstexprDtor() {}
+};
+
+union UnionWithNonLiteralMember {
+  HasDefaultCtorAndNonConstexprDtor x;
+  int y;
+
+  constexpr UnionWithNonLiteralMember() : x{} {}
+};
+#if __cplusplus >= 202002L
+static_assert(__is_literal(UnionWithNonLiteralMember), "fail");
+#else
+static_assert(!__is_literal(UnionWithNonLiteralMember), "fail");
+#endif
+
+union UnionWithNonLiteralMemberExplicitDtor1 {
+  HasDefaultCtorAndNonConstexprDtor x;
+  int y;
+  // expected-note@-2 {{destructor of 'UnionWithNonLiteralMemberExplicitDtor1' is implicitly deleted because variant field 'x' has a non-trivial destructor}}
+
+  constexpr UnionWithNonLiteralMemberExplicitDtor1() : x{} {}
+  ~UnionWithNonLiteralMemberExplicitDtor1() = default; // expected-warning {{explicitly defaulted destructor is implicitly deleted}}
+  // expected-note@-1 {{replace 'default' with 'delete'}}
+};
+#if __cplusplus >= 202002L
+static_assert(__is_literal(UnionWithNonLiteralMemberExplicitDtor1), "fail");
+#else
+static_assert(!__is_literal(UnionWithNonLiteralMemberExplicitDtor1), "fail");
+#endif
+
+union UnionWithNonLiteralMemberExplicitDtor2 {
+  HasDefaultCtorAndNonConstexprDtor x;
+  int y;
+
+  constexpr UnionWithNonLiteralMemberExplicitDtor2() : x{} {}
+  ~UnionWithNonLiteralMemberExplicitDtor2() = delete;
+};
+static_assert(!__is_literal(UnionWithNonLiteralMemberExplicitDtor2), "fail");
+
+#if __cplusplus >= 202002L
+union UnionWithNonLiteralMemberConstexprDtor1 {
+  HasDefaultCtorAndNonConstexprDtor x;
+  int y;
+  // expected-note@-2 {{destructor of 'UnionWithNonLiteralMemberConstexprDtor1' is implicitly deleted because variant field 'x' has a non-trivial destructor}}
+
+  constexpr UnionWithNonLiteralMemberConstexprDtor1() : x{} {}
+  constexpr ~UnionWithNonLiteralMemberConstexprDtor1() = default; // expected-warning {{explicitly defaulted destructor is implicitly deleted}}
+  // expected-note@-1 {{replace 'default' with 'delete'}}
+};
+static_assert(__is_literal(UnionWithNonLiteralMemberConstexprDtor1), "fail");
+
+union UnionWithNonLiteralMemberConstexprDtor2 {
+  HasDefaultCtorAndNonConstexprDtor x;
+  int y;
+
+  constexpr UnionWithNonLiteralMemberConstexprDtor2() : x{} {}
+  constexpr ~UnionWithNonLiteralMemberConstexprDtor2() = delete;
+};
+static_assert(__is_literal(UnionWithNonLiteralMemberConstexprDtor2), "fail");
+#endif
+}
+#endif