[AST] Add a way to erase marker protocols from a protocol composition

Author: xedin

include/swift/AST/Types.h

@@ -6050,6 +6050,10 @@ class ProtocolCompositionType final : public TypeBase,
     return Bits.ProtocolCompositionType.HasExplicitAnyObject;
   }
 
+  /// Produce a new type (potentially not be a protoocl composition)
+  /// which drops all of the marker protocol types associated with this one.
+  Type withoutMarkerProtocols() const;
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const TypeBase *T) {
     return T->getKind() == TypeKind::ProtocolComposition;

lib/AST/Type.cpp

@@ -3704,6 +3704,20 @@ Type ProtocolCompositionType::getInverseOf(const ASTContext &C,
                                       /*HasExplicitAnyObject=*/false);
 }
 
+Type ProtocolCompositionType::withoutMarkerProtocols() const {
+  SmallVector<Type, 4> newMembers;
+  llvm::copy_if(getMembers(), std::back_inserter(newMembers), [](Type member) {
+    auto *P = member->getAs<ProtocolType>();
+    return !(P && P->getDecl()->isMarkerProtocol());
+  });
+
+  if (newMembers.size() == getMembers().size())
+    return Type(const_cast<ProtocolCompositionType *>(this));
+
+  return ProtocolCompositionType::get(getASTContext(), newMembers,
+                                      getInverses(), hasExplicitAnyObject());
+}
+
 Type ProtocolCompositionType::get(const ASTContext &C,
                                   ArrayRef<Type> Members,
                                   InvertibleProtocolSet Inverses,