Marker protocols are self-conforming.

Author: DougGregor

lib/AST/Module.cpp

@@ -918,8 +918,8 @@ ModuleDecl::lookupExistentialConformance(Type type, ProtocolDecl *protocol) {
 
   // Due to an IRGen limitation, witness tables cannot be passed from an
   // existential to an archetype parameter, so for now we restrict this to
-  // @objc protocols.
-  if (!layout.isObjC()) {
+  // @objc protocols and marker protocols.
+  if (!layout.isObjC() && !protocol->isMarkerProtocol()) {
     // There's a specific exception for protocols with self-conforming
     // witness tables, but the existential has to be *exactly* that type.
     // TODO: synthesize witness tables on-demand for protocol compositions

lib/Sema/TypeCheckDecl.cpp

@@ -643,8 +643,12 @@ ProtocolRequiresClassRequest::evaluate(Evaluator &evaluator,
 bool
 ExistentialConformsToSelfRequest::evaluate(Evaluator &evaluator,
                                            ProtocolDecl *decl) const {
-  // If it's not @objc, it conforms to itself only if it has a self-conformance
-  // witness table.
+  // Marker protocols always self-conform.
+  if (decl->isMarkerProtocol())
+    return true;
+
+  // Otherwise, if it's not @objc, it conforms to itself only if it has a
+  // self-conformance witness table.
   if (!decl->isObjC())
     return decl->requiresSelfConformanceWitnessTable();
 

test/attr/attr_marker_protocol.swift

@@ -30,10 +30,12 @@ protocol P6: P3 { } // okay
 
 func genericOk<T: P3>(_: T) { }
 
-func testGenericOk(i: Int, arr: [Int], nope: [Double]) {
+func testGenericOk(i: Int, arr: [Int], nope: [Double], p3: P3, p3array: [P3]) {
   genericOk(i)
   genericOk(arr)
   genericOk(nope) // expected-error{{global function 'genericOk' requires that 'Double' conform to 'P3'}}
+  genericOk(p3)
+  genericOk(p3array)
 }
 
 // Incorrect uses of marker protocols in types.