RequirementMachine: Remove unsound cycle-breaking hack

Author: slavapestov

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -292,12 +292,6 @@ struct InferRequirementsWalker : public TypeWalker {
     auto decl = ty->getAnyNominal();
     if (!decl) return Action::Continue;
 
-    // FIXME: The GSB and the request evaluator both detect a cycle here if we
-    // force a recursive generic signature.  We should look into moving cycle
-    // detection into the generic signature request(s) - see rdar://55263708
-    if (!decl->hasComputedGenericSignature())
-      return Action::Continue;
-
     auto genericSig = decl->getGenericSignature();
     if (!genericSig)
       return Action::Continue;

test/Generics/requirement_inference_funny_order.swift

@@ -0,0 +1,25 @@
+// RUN: %target-typecheck-verify-swift -requirement-machine-inferred-signatures=on
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+
+protocol P1 {}
+
+protocol P2 {}
+
+protocol IteratorProtocol {
+  associatedtype Element
+
+  func next() -> Element?
+}
+
+// CHECK: requirement_inference_funny_order.(file).LocalArray@
+// CHECK: Generic signature: <Element where Element : P1>
+
+// CHECK: ExtensionDecl line={{[0-9]+}} base=LocalArray
+// CHECK: Generic signature: <Element where Element : P1, Element : P2>
+extension LocalArray where Element : P2 {
+  static func ==(lhs: Self, rhs: Self) -> Bool {}
+}
+
+struct LocalArray<Element : P1>: IteratorProtocol {
+  func next() -> Element? {}
+}