RequirementMachine: Subtle fix for minimal conformances algorithm

We don't need to be able to recover the parent type of a conformance
rule if the rule itself is redundant.

Author: slavapestov

lib/AST/RequirementMachine/MinimalConformances.cpp

@@ -655,21 +655,21 @@ bool MinimalConformances::isValidConformancePath(
 
       if (!foundValidConformancePath)
         return false;
-    }
-
-    auto found = ParentPaths.find(ruleID);
-    if (found != ParentPaths.end()) {
-      SWIFT_DEFER {
-        visited.erase(ruleID);
-      };
-      visited.insert(ruleID);
-
-      // If 'req' is based on some other conformance requirement
-      // `T.[P.]A : Q', we want to make sure that we have a
-      // non-redundant derivation for 'T : P'.
-      if (!isValidConformancePath(visited, found->second,
-                                  /*allowConcrete=*/false)) {
-        return false;
+    } else {
+      auto found = ParentPaths.find(ruleID);
+      if (found != ParentPaths.end()) {
+        SWIFT_DEFER {
+          visited.erase(ruleID);
+        };
+        visited.insert(ruleID);
+
+        // If 'req' is based on some other conformance requirement
+        // `T.[P.]A : Q', we want to make sure that we have a
+        // non-redundant derivation for 'T : P'.
+        if (!isValidConformancePath(visited, found->second,
+                                    /*allowConcrete=*/false)) {
+          return false;
+        }
       }
     }
   }

test/Generics/redundant_parent_path_in_protocol.swift

@@ -0,0 +1,50 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=on 2>&1 | %FileCheck %s
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P1@
+// CHECK-NEXT: Requirement signature: <Self>
+
+protocol P1 {}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P2@
+// CHECK-NEXT: Requirement signature: <Self where Self.A : P1, Self.B : P2>
+
+protocol P2 {
+  associatedtype A: P1
+  associatedtype B: P2
+}
+
+struct Concrete: P1, P2 {
+  typealias A = Concrete
+  typealias B = Concrete
+}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P3a@
+// CHECK-NEXT: Requirement signature: <Self where Self.T : P2>
+
+protocol P3a {
+  associatedtype T : P2
+}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P3b@
+// CHECK-NEXT: Requirement signature: <Self where Self.T : P2, Self.T.A == Self.T.B>
+
+protocol P3b {
+  associatedtype T : P2 where T.A == T.B
+}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P4a@
+// CHECK-NEXT: Requirement signature: <Self where Self : P3a, Self.T == Concrete>
+
+protocol P4a : P3a where T.A == T.B, T == Concrete {}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P4b@
+// CHECK-NEXT: Requirement signature: <Self where Self : P3b, Self.T == Concrete>
+
+protocol P4b : P3b where T == Concrete {}
+
+// CHECK-LABEL: redundant_parent_path_in_protocol.(file).P5@
+// CHECK-NEXT: Requirement signature: <Self where Self.T == Concrete>
+
+protocol P5 {
+  associatedtype T : P2 where T.A == T.B, T == Concrete
+}