[GSB] Install basic recursion checking.

Recursive protocol conformances still aren't functional, so reinstate
some checking for recursive protocol conformances. It doesn't catch
everything that the previous version did---but we don't crash on such
cases anymore, either.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -2355,21 +2355,6 @@ ConstraintResult GenericSignatureBuilder::addConformanceRequirement(
   if (!PAT->addConformance(Proto, Source, *this))
     return ConstraintResult::Resolved;
 
-#if false
-  // FIXME: Ad hoc recursion breaking.
-  if (Visited.count(Proto)) {
-    markPotentialArchetypeRecursive(PAT, Proto, Source);
-    return ConstraintResult::Conflicting;
-  }
-
-  bool inserted = Visited.insert(Proto).second;
-  assert(inserted);
-  (void) inserted;
-  SWIFT_DEFER {
-    Visited.erase(Proto);
-  };
-#endif
-
   auto concreteSelf = PAT->getDependentType({}, /*allowUnresolved=*/true);
   auto protocolSubMap = SubstitutionMap::getProtocolSubstitutions(
       Proto, concreteSelf, ProtocolConformanceRef(Proto));
@@ -3091,6 +3076,23 @@ ConstraintResult GenericSignatureBuilder::addInheritedRequirements(
                         getFloatingSource(typeRepr, /*forInferred=*/true));
     }
 
+    // Check for direct recursion.
+    if (auto assocType = dyn_cast<AssociatedTypeDecl>(decl)) {
+      auto proto = assocType->getProtocol();
+      if (auto inheritedProto = inheritedType->getAs<ProtocolType>()) {
+        if (inheritedProto->getDecl() == proto ||
+            inheritedProto->getDecl()->inheritsFrom(proto)) {
+          auto source = getFloatingSource(typeRepr, /*forInferred=*/false);
+          if (auto resolved = resolve(type, source)) {
+            if (auto pa = resolved->getPotentialArchetype()) {
+              markPotentialArchetypeRecursive(pa, proto, source.getSource(pa));
+              return ConstraintResult::Conflicting;
+            }
+          }
+        }
+      }
+    }
+
     return addTypeRequirement(type, inheritedType,
                               getFloatingSource(typeRepr,
                                                 /*forInferred=*/false),

test/decl/protocol/recursive_requirement.swift

@@ -41,7 +41,7 @@ struct Y2 : P2 {
 }
 
 func f<T : P2>(_ z: T) {
- _ = X2<T.A>()
+ _ = X2<T.A>() // expected-error{{type 'T.A' does not conform to protocol 'P2'}}
 }
 
 // -----
@@ -69,11 +69,11 @@ f2(Y3())
 // -----
 
 protocol Alpha {
-  associatedtype Beta: Gamma // expected-error{{type may not reference itself as a requirement}}
+  associatedtype Beta: Gamma
 }
 
 protocol Gamma {
-  associatedtype Delta: Alpha // expected-error{{type may not reference itself as a requirement}}
+  associatedtype Delta: Alpha
 }
 
 // FIXME: Redundancy diagnostics are odd here.
@@ -111,7 +111,7 @@ protocol AsExistentialAssocTypeAgainB {
 
 // SR-547
 protocol A {
-    associatedtype B1: B // expected-error{{type may not reference itself as a requirement}}
+    associatedtype B1: B
     associatedtype C1: C
 
     mutating func addObserver(_ observer: B1, forProperty: C1)
@@ -122,7 +122,7 @@ protocol C {
 }
 
 protocol B {
-    associatedtype BA: A // expected-error{{type may not reference itself as a requirement}}
+    associatedtype BA: A
     associatedtype BC: C
 
     func observeChangeOfProperty(_ property: BC, observable: BA)