[CSSimplify] Skip member access on existential checks if base is existential only due to marker protocols

If the base type is composed with marker protocol(s) i.e.
`<<Type>> & Sendable`, let's skip this check because such
compositions are always opened and simplified down to a
superclass bound post-Sema.

Resolves: rdar://148782046

lib/Sema/CSSimplify.cpp

@@ -10431,22 +10431,33 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     }
 
     const auto isUnsupportedExistentialMemberAccess = [&] {
-      // We may not be able to derive a well defined type for an existential
-      // member access if the member's signature references 'Self'.
-      if (instanceTy->isExistentialType()) {
-        switch (isMemberAvailableOnExistential(instanceTy, decl)) {
-        case ExistentialMemberAccessLimitation::Unsupported:
-        // TODO: Write-only accesses are not supported yet.
-        case ExistentialMemberAccessLimitation::WriteOnly:
-          return true;
+      if (!instanceTy->isExistentialType())
+        return false;
 
-        case ExistentialMemberAccessLimitation::ReadOnly:
-        case ExistentialMemberAccessLimitation::None:
-          break;
-        }
+      // If the base type is composed with marker protocol(s) i.e.
+      // `<<Type>> & Sendable`, let's skip this check because such
+      // compositions are always opened and simplified down to a
+      // superclass bound post-Sema.
+      if (auto *existential = instanceTy->getAs<ExistentialType>()) {
+        auto *compositionTy =
+            existential->getConstraintType()->getAs<ProtocolCompositionType>();
+        if (compositionTy &&
+            !compositionTy->withoutMarkerProtocols()->isExistentialType())
+          return false;
       }
 
-      return false;
+      // We may not be able to derive a well defined type for an existential
+      // member access if the member's signature references 'Self'.
+      switch (isMemberAvailableOnExistential(instanceTy, decl)) {
+      case ExistentialMemberAccessLimitation::Unsupported:
+      // TODO: Write-only accesses are not supported yet.
+      case ExistentialMemberAccessLimitation::WriteOnly:
+        return true;
+
+      case ExistentialMemberAccessLimitation::ReadOnly:
+      case ExistentialMemberAccessLimitation::None:
+        return false;
+      }
     };
 
     // See if we have an instance method, instance member or static method,

test/Interpreter/protocol_composition_with_markers.swift

@@ -58,3 +58,27 @@ do {
   generic(Int.self)
   // CHECK: D<Int>
 }
+
+protocol Q {
+  func update(_: [Self])
+}
+
+extension Q {
+  func update(_ arr: [Self]) { print(Self.self) }
+}
+
+do {
+  class Parent : Q {}
+  class Subclass: Parent {}
+
+  func test(_ v: Parent & Sendable) {
+    v.update([])
+  }
+
+  test(Subclass())
+  // CHECK: Subclass
+
+  let sendableV: any Subclass & Sendable = Subclass()
+  test(sendableV)
+  // CHECK: Subclass
+}