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

[shafik]

In 765d8a1 we impelemented a fix for incorrect deletion of default constructors in unions. This fix missed a case and so this PR will extend the fix to cover the additional case.

Fixes: #81774

[llvmbot]

@llvm/pr-subscribers-clang

Author: Shafik Yaghmour (shafik)

Changes

In 765d8a1 we impelemented a fix for incorrect deletion of default constructors in unions. This fix missed a case and so this PR will extend the fix to cover the additional case.

Fixes: #81774

---

Full diff: https://github.com/llvm/llvm-project/pull/82407.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/Sema/SemaDeclCXX.cpp (+15-3)
• (modified) clang/test/CodeGen/union-non-trivial-member.cpp (+17)
• (modified) clang/test/SemaCXX/cxx0x-nontrivial-union.cpp (+11)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5bca2c965c866b..452382eb6c4a1e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -292,6 +292,9 @@ Bug Fixes to C++ Support
   was only accepted at namespace scope but not at local function scope.
 - Clang no longer tries to call consteval constructors at runtime when they appear in a member initializer.
   (`#782154 <https://github.com/llvm/llvm-project/issues/82154>`_`)
+- 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 79263bc3ff671d..25a4b4381ca25e 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/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;
diff --git a/clang/test/CodeGen/union-non-trivial-member.cpp b/clang/test/CodeGen/union-non-trivial-member.cpp
index fdc9fd16911e14..8b055a9970fc75 100644
--- a/clang/test/CodeGen/union-non-trivial-member.cpp
+++ b/clang/test/CodeGen/union-non-trivial-member.cpp
@@ -15,14 +15,25 @@ 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
@@ -30,8 +41,14 @@ void f() {
 // 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
diff --git a/clang/test/SemaCXX/cxx0x-nontrivial-union.cpp b/clang/test/SemaCXX/cxx0x-nontrivial-union.cpp
index c7cdf76d850dbe..141fe96f512471 100644
--- a/clang/test/SemaCXX/cxx0x-nontrivial-union.cpp
+++ b/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 b/c NoState has a brace-or-equal-initializer
+union a {
+        int NoState = 0;
+        Handle CustomState;
+} b;
+} // namespace GH81774

[MitalAshok]

This is CWG2084. Could you also add some tests to CXX/drs/ so that www/cxx_dr_status.html can be updated?

[cor3ntin]

This is CWG2084. Could you also add some tests to CXX/drs/ so that www/cxx_dr_status.html can be updated?

Can you address that? thanks!

[cor3ntin]

Thanks!

[void2012]

@shafik Can this be safely backported to the upcoming LLVM 21.x releases?