SIL: Bridge 'any Sendable' via the id-as-Any path

include/swift/AST/Types.h

@@ -601,6 +601,9 @@ class alignas(1 << TypeAlignInBits) TypeBase
   /// Is this the 'Any' type?
   bool isAny();
 
+  /// Is this an existential containing only marker protocols?
+  bool isMarkerExistential();
+
   bool isPlaceholder();
 
   /// Returns true if this is a move-only type.

lib/AST/Type.cpp

@@ -166,6 +166,28 @@ bool TypeBase::isAny() {
   return constraint->isEqual(getASTContext().TheAnyType);
 }
 
+bool TypeBase::isMarkerExistential() {
+  Type constraint = this;
+  if (auto existential = constraint->getAs<ExistentialType>())
+    constraint = existential->getConstraintType();
+
+  if (!constraint->isConstraintType())
+    return false;
+
+  auto layout = constraint->getExistentialLayout();
+  if (layout.hasExplicitAnyObject ||
+      layout.explicitSuperclass) {
+    return false;
+  }
+
+  for (auto *proto : layout.getProtocols()) {
+    if (!proto->isMarkerProtocol())
+      return false;
+  }
+
+  return true;
+}
+
 bool TypeBase::isPureMoveOnly() {
   if (auto *nom = getNominalOrBoundGenericNominal())
     return nom->isMoveOnly();
@@ -2914,8 +2936,8 @@ getObjCObjectRepresentable(Type type, const DeclContext *dc) {
       return ForeignRepresentableKind::Object;
   }
 
-  // Any can be bridged to id.
-  if (type->isAny()) {
+  // Existentials consisting of only marker protocols can be bridged to id.
+  if (type->isMarkerExistential()) {
     return ForeignRepresentableKind::Bridged;
   }
 

lib/SIL/IR/Bridging.cpp

@@ -172,8 +172,9 @@ Type TypeConverter::getLoweredCBridgedType(AbstractionPattern pattern,
     }
   }
 
-  // `Any` can bridge to `AnyObject` (`id` in ObjC).
-  if (t->isAny())
+  // Existentials consisting of only marker protocols can bridge to
+  // `AnyObject` (`id` in ObjC).
+  if (t->isMarkerExistential())
     return Context.getAnyObjectType();
 
   if (auto funTy = t->getAs<FunctionType>()) {

test/SILGen/Inputs/objc_bridging_sendable.h

@@ -0,0 +1,5 @@
+@import Foundation;
+
+@interface NSBlah: NSObject
+- (void) takeSendable: (id __attribute__((swift_attr("@Sendable")))) x;
+@end

test/SILGen/objc_bridging_sendable.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) -import-objc-header %S/Inputs/objc_bridging_sendable.h %s | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+// CHECK-LABEL: sil [ossa] @$s22objc_bridging_sendable18passSendableToObjCyys0E0_pF : $@convention(thin) (@in_guaranteed any Sendable) -> () {
+// CHECK:  function_ref @$ss27_bridgeAnythingToObjectiveCyyXlxlF : $@convention(thin) <τ_0_0> (@in_guaranteed τ_0_0) -> @owned AnyObject
+// CHECK: objc_method {{%.*}} : $NSBlah, #NSBlah.takeSendable!foreign : (NSBlah) -> ((any Sendable)?) -> (), $@convention(objc_method) (Optional<AnyObject>, NSBlah) -> ()
+// CHECK: return
+public func passSendableToObjC(_ s: Sendable) {
+  NSBlah().takeSendable(s)
+}