GSB/RequirementMachine: Remove unsound cycle-breaking hack

lib/AST/GenericSignatureBuilder.cpp

@@ -5209,12 +5209,6 @@ class GenericSignatureBuilder::InferRequirementsWalker : public TypeWalker {
     // Infer from generic nominal types.
     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)

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/generic_types.swift

@@ -230,7 +230,7 @@ class Top {}
 class Bottom<T : Bottom<Top>> {}
 // expected-error@-1 {{'Bottom' requires that 'Top' inherit from 'Bottom<Top>'}}
 // expected-note@-2 {{requirement specified as 'T' : 'Bottom<Top>' [with T = Top]}}
-// expected-error@-3 {{generic class 'Bottom' has self-referential generic requirements}}
+// expected-error@-3 4{{generic class 'Bottom' has self-referential generic requirements}}
 // expected-note@-4 {{while resolving type 'Bottom<Top>'}}
 // expected-note@-5 {{through reference here}}
 

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? {}
+}

test/decl/protocol/req/recursion.swift

@@ -10,6 +10,7 @@ extension SomeProtocol where T == Optional<T> { } // expected-error{{same-type c
 
 class X<T> where T == X { // expected-error{{same-type constraint 'T' == 'X<T>' is recursive}}
 // expected-error@-1{{same-type requirement makes generic parameter 'T' non-generic}}
+// expected-error@-2 3{{generic class 'X' has self-referential generic requirements}}
     var type: T { return Swift.type(of: self) } // expected-error{{cannot convert return expression of type 'X<T>.Type' to return type 'T'}}
 }
 
@@ -44,7 +45,7 @@ public protocol P {
 }
 
 public struct S<A: P> where A.T == S<A> {
-// expected-error@-1 {{generic struct 'S' has self-referential generic requirements}}
+// expected-error@-1 4{{generic struct 'S' has self-referential generic requirements}}
 // expected-note@-2 {{while resolving type 'S<A>'}}
   func f(a: A.T) {
     g(a: id(t: a)) // `a` has error type which is diagnosed as circular reference
@@ -70,7 +71,7 @@ protocol PI {
 }
 
 struct SI<A: PI> : I where A : I, A.T == SI<A> {
-// expected-error@-1 {{generic struct 'SI' has self-referential generic requirements}}
+// expected-error@-1 4{{generic struct 'SI' has self-referential generic requirements}}
 // expected-note@-2 {{while resolving type 'SI<A>'}}
   func ggg<T : I>(t: T.Type) -> T {
     return T()
@@ -98,7 +99,9 @@ struct S5<A: PI> : I where A : I, A.T == S4<A> { }
 
 // Used to hit ArchetypeBuilder assertions
 struct SU<A: P> where A.T == SU {
+// expected-error@-1 3{{generic struct 'SU' has self-referential generic requirements}}
 }
 
 struct SIU<A: PI> : I where A : I, A.T == SIU {
+// expected-error@-1 3{{generic struct 'SIU' has self-referential generic requirements}}
 }

validation-test/compiler_crashers_2_fixed/0064-rdar27627862.swift

@@ -1,6 +1,7 @@
-// RUN: %target-swift-frontend %s -emit-ir
+// RUN: %target-typecheck-verify-swift
 
 enum Term<S> where S: Sequence, S.Iterator.Element == Term {
+// expected-error@-1 3{{generic enum 'Term' has self-referential generic requirements}}
     case Cons(head: String, tail: S)
 }