[Clang][Sema] Fix incorrect rejection default construction of union with nontrivial member

clang/docs/ReleaseNotes.rst

@@ -298,6 +298,9 @@ Bug Fixes to C++ Support
   Fixes (`#82941 <https://github.com/llvm/llvm-project/issues/82941>`_),
   (`#42411 <https://github.com/llvm/llvm-project/issues/42411>`_), and
   (`#18121 <https://github.com/llvm/llvm-project/issues/18121>`_).
+- Fix for clang incorrectly rejecting the default construction of a union with
+  nontrivial member when another member has an initializer.
+  (`#81774 <https://github.com/llvm/llvm-project/issues/81774>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDeclCXX.cpp

@@ -9442,9 +9442,21 @@ bool SpecialMemberDeletionInfo::shouldDeleteForSubobjectCall(
 
   int DiagKind = -1;
 
-  if (SMOR.getKind() == Sema::SpecialMemberOverloadResult::NoMemberOrDeleted)
-    DiagKind = !Decl ? 0 : 1;
-  else if (SMOR.getKind() == Sema::SpecialMemberOverloadResult::Ambiguous)
+  if (SMOR.getKind() == Sema::SpecialMemberOverloadResult::NoMemberOrDeleted) {
+    if (CSM == Sema::CXXDefaultConstructor && Field &&
+        Field->getParent()->isUnion()) {
+      // [class.default.ctor]p2:
+      //   A defaulted default constructor for class X is defined as deleted if
+      //   - X is a union that has a variant member with a non-trivial default
+      //     constructor and no variant member of X has a default member
+      //     initializer
+      const auto *RD = cast<CXXRecordDecl>(Field->getParent());
+      if (!RD->hasInClassInitializer())
+        DiagKind = !Decl ? 0 : 1;
+    } else {
+      DiagKind = !Decl ? 0 : 1;
+    }
+  } else if (SMOR.getKind() == Sema::SpecialMemberOverloadResult::Ambiguous)
     DiagKind = 2;
   else if (!isAccessible(Subobj, Decl))
     DiagKind = 3;

clang/test/CodeGen/union-non-trivial-member.cpp

@@ -15,23 +15,40 @@ union UnionNonTrivial {
     non_trivial_constructor b{};
 };
 
+struct Handle {
+    Handle(int) {}
+};
+
+union UnionNonTrivialEqualInit {
+    int NoState = 0;
+    Handle CustomState;
+};
+
 void f() {
     UnionInt u1;
     UnionNonTrivial u2;
+    UnionNonTrivialEqualInit u3;
 }
 
 // CHECK:      define dso_local void @_Z1fv()
 // CHECK:        call void @_ZN8UnionIntC1Ev
 // CHECK-NEXT:   call void @_ZN15UnionNonTrivialC1Ev
+// CHECK-NEXT:   call void @_ZN24UnionNonTrivialEqualInitC1Ev
 
 // CHECK:      define {{.*}}void @_ZN8UnionIntC1Ev
 // CHECK:        call void @_ZN8UnionIntC2Ev
 
 // CHECK:      define {{.*}}void @_ZN15UnionNonTrivialC1Ev
 // CHECK:        call void @_ZN15UnionNonTrivialC2Ev
 
+// CHECK:      define {{.*}}void @_ZN24UnionNonTrivialEqualInitC1Ev
+// CHECK:        call void @_ZN24UnionNonTrivialEqualInitC2Ev
+
 // CHECK:      define {{.*}}void @_ZN8UnionIntC2Ev
 // CHECK:        store i32 1000
 
 // CHECK:      define {{.*}}void @_ZN15UnionNonTrivialC2Ev
 // CHECK:        call void @_ZN23non_trivial_constructorC1Ev
+
+// CHECK:      define {{.*}}void @_ZN24UnionNonTrivialEqualInitC2Ev
+// CHECK:        store i32 0

clang/test/SemaCXX/cxx0x-nontrivial-union.cpp

@@ -188,3 +188,14 @@ static_assert(U2().b.x == 100, "");
 static_assert(U3().b.x == 100, "");
 
 } // namespace GH48416
+
+namespace GH81774 {
+struct Handle {
+    Handle(int) {}
+};
+// Should be well-formed because NoState has a brace-or-equal-initializer.
+union a {
+        int NoState = 0;
+        Handle CustomState;
+} b;
+} // namespace GH81774