RequirementMachine: Fix concrete conformances with inherited protocols

Author: slavapestov

lib/AST/RequirementMachine/GeneratingConformances.cpp

@@ -815,7 +815,12 @@ void GeneratingConformances::computeGeneratingConformances(
       bool derivedViaConcrete = false;
       for (const auto &path : paths) {
         if (path.empty())
-          break;
+          continue;
+
+        // If the rule is itself a concrete conformance, it is not
+        // derived-via-concrete via itself.
+        if (path.size() == 1 && path.front() == ruleID)
+          continue;
 
         if (System.getRule(path.back()).getLHS().back().getKind() ==
             Symbol::Kind::ConcreteConformance) {

test/Generics/inherited_concrete_conformance_in_protocol.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=on 2>&1 | %FileCheck %s
+
+protocol Sequence {}
+
+protocol Collection : Sequence {}
+
+struct MyCollection : Collection {}
+
+// CHECK-LABEL: inherited_concrete_conformance_in_protocol.(file).P@
+// CHECK-LABEL: Requirement signature: <Self where Self.T == MyCollection>
+
+protocol P {
+  associatedtype T : Collection where T == MyCollection
+}