[AST] GenericSignature: Add a way to drop marker protocol requirements

Author: xedin

include/swift/AST/GenericSignature.h

@@ -122,6 +122,10 @@ class GenericSignature {
                               ArrayRef<Requirement> requirements,
                               bool isKnownCanonical = false);
 
+  /// Produce a new generic signature which drops all of the marker
+  /// protocol conformance requirements associated with this one.
+  GenericSignature withoutMarkerProtocols() const;
+
 public:
   static ASTContext &getASTContext(TypeArrayView<GenericTypeParamType> params,
                                    ArrayRef<Requirement> requirements);

lib/AST/GenericSignature.cpp

@@ -1163,3 +1163,24 @@ swift::buildGenericSignature(ASTContext &ctx,
         addedRequirements},
       GenericSignatureWithError()).getPointer();
 }
+
+GenericSignature GenericSignature::withoutMarkerProtocols() const {
+  auto requirements = getRequirements();
+  SmallVector<Requirement, 4> reducedRequirements;
+
+  // Drop all conformance requirements to marker protocols (if any).
+  llvm::copy_if(requirements, std::back_inserter(reducedRequirements),
+                [](const Requirement &requirement) {
+                  if (requirement.getKind() == RequirementKind::Conformance) {
+                    auto *protocol = requirement.getProtocolDecl();
+                    return !protocol->isMarkerProtocol();
+                  }
+                  return true;
+                });
+
+  // If nothing changed, let's return this signature back.
+  if (requirements.size() == reducedRequirements.size())
+    return *this;
+
+  return GenericSignature::get(getGenericParams(), reducedRequirements);
+}