RequirementMachine: Fix for redundant protocol refinement via concrete type requirements

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -98,7 +98,8 @@ bool Rule::isProtocolRefinementRule() const {
       RHS.size() == 1 &&
       LHS[0] == RHS[0] &&
       LHS[0].getKind() == Symbol::Kind::Protocol &&
-      LHS[1].getKind() == Symbol::Kind::Protocol &&
+      (LHS[1].getKind() == Symbol::Kind::Protocol ||
+       LHS[1].getKind() == Symbol::Kind::ConcreteConformance) &&
       LHS[0] != LHS[1]) {
 
     // A protocol refinement rule must be from a directly-stated

test/Generics/redundant_protocol_inheritance_via_concrete.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=on 2>&1 | %FileCheck %s
+
+protocol P {}
+class C : P {}
+
+// CHECK-LABEL: redundant_protocol_inheritance_via_concrete.(file).Q@
+// CHECK-NEXT: Requirement signature: <Self where Self : C>
+protocol Q : P, C {}