From e0487b6ce053c9054db8c0e02b72e3d9a9605f14 Mon Sep 17 00:00:00 2001
From: Nate Chandler <nate_chandler@apple.com>
Date: Thu, 8 Feb 2024 11:34:55 -0800
Subject: [PATCH] [BitwiseCopyable] Ban non-trivial C++ types.

Such types can't be copied bit-by-bit.
---
 include/swift/AST/DiagnosticsSema.def | 2 ++
 lib/Sema/TypeCheckBitwise.cpp         | 8 ++++++++
 2 files changed, 10 insertions(+)

diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index a9425b50c89cc..5304c5219f592 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -7619,6 +7619,8 @@ ERROR(non_bitwise_copyable_type_noncopyable,none,
       "noncopyable type cannot conform to 'BitwiseCopyable'", ())
 ERROR(non_bitwise_copyable_type_nonescapable,none,
       "nonescapable type cannot conform to 'BitwiseCopyable'", ())
+ERROR(non_bitwise_copyable_type_cxx_nontrivial,none,
+      "non-trivial C++ type cannot conform to 'BitwiseCopyable'", ())
 ERROR(non_bitwise_copyable_type_member,none,
       "%select{stored property %2|associated value %2}1 of "
       "'BitwiseCopyable'-conforming %kind3 has non-bitwise-copyable type %0",
diff --git a/lib/Sema/TypeCheckBitwise.cpp b/lib/Sema/TypeCheckBitwise.cpp
index 149a96c423223..ed384bd267652 100644
--- a/lib/Sema/TypeCheckBitwise.cpp
+++ b/lib/Sema/TypeCheckBitwise.cpp
@@ -262,6 +262,14 @@ static bool checkBitwiseCopyableInstanceStorage(NominalTypeDecl *nominal,
     return true;
   }
 
+  auto *sd = dyn_cast<StructDecl>(nominal);
+  if (sd && sd->isCxxNonTrivial()) {
+    if (!isImplicit(check)) {
+      nominal->diagnose(diag::non_bitwise_copyable_type_cxx_nontrivial);
+    }
+    return true;
+  }
+
   BitwiseCopyableStorageVisitor visitor(nominal, dc, check);
 
   return visitor.visit(nominal, dc) || visitor.invalid;