RequirementMachine: Don't call ProtocolDecl::getSuperclass() in concrete contraction

This changes the minimized signature in a very narrow edge case.

If you have

    class C : P {}

and also

    protocol P : C {}
    protocol P where Self : C {}

then <T where T : P, T : C> now becomes <T : P> both with spellings;
before, the first one gave <T : P> and the second <T : C>.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -624,19 +624,26 @@ bool ConcreteContraction::performConcreteContraction(
 
     auto superclassTy = *found->second.begin();
 
-    for (const auto *proto : pair.second) {
-      if (auto otherSuperclassTy = proto->getSuperclass()) {
-        if (Debug) {
-          llvm::dbgs() << "@ Subject type of superclass requirement "
-                       << subjectType << " : " << superclassTy
-                       << " conforms to "<< proto->getName()
-                       << " which has a superclass bound "
-                       << otherSuperclassTy << "\n";
-        }
-
-        if (superclassTy->isEqual(otherSuperclassTy)) {
-          Superclasses.erase(subjectType);
-          break;
+    for (auto *proto : pair.second) {
+      auto *module = proto->getParentModule();
+      if (module->lookupConformance(superclassTy, proto)) {
+        auto genericSig = proto->getGenericSignature();
+        // FIXME: If we end up here while building the requirement
+        // signature of `proto`, we will hit a request cycle.
+        if (auto otherSuperclassTy = genericSig->getSuperclassBound(
+                proto->getSelfInterfaceType())) {
+          if (Debug) {
+            llvm::dbgs() << "@ Subject type of superclass requirement "
+                         << subjectType << " : " << superclassTy
+                         << " conforms to "<< proto->getName()
+                         << " which has a superclass bound "
+                         << otherSuperclassTy << "\n";
+          }
+
+          if (superclassTy->isEqual(otherSuperclassTy)) {
+            Superclasses.erase(subjectType);
+            break;
+          }
         }
       }
     }

test/Generics/protocol_superclass_cycle.swift

@@ -52,3 +52,9 @@ func foo5<T : SubSuperP>(_: T) {}
 // CHECK-NEXT: Generic signature: <T where T : Q, T.[Q]T : SubSuperP>
 func foo6<T : Q>(_: T) where T.T : SubSuperP {}
 
+protocol POther where Self : COther {}
+class COther : POther {}
+
+// CHECK-LABEL: .foo7@
+// CHECK-NEXT: Generic signature: <T where T : POther>
+func foo7<T>(_: T) where T : COther, T : POther {}