SE-0157: Enable recursive protocol constraints by default.

We don't know if they work in general, but specific cases of recursive
protocol constraints are working, and enabling them doesn't break
anything.

include/swift/AST/DiagnosticsSema.def

@@ -1668,8 +1668,6 @@ ERROR(requires_generic_param_made_equal_to_concrete,none,
       (Type))
 ERROR(recursive_type_reference,none,
       "%0 %1 references itself", (DescriptiveDeclKind, Identifier))
-ERROR(recursive_requirement_reference,none,
-      "type may not reference itself as a requirement",())
 ERROR(recursive_same_type_constraint,none,
       "same-type constraint %0 == %1 is recursive", (Type, Type))
 ERROR(recursive_superclass_constraint,none,

include/swift/AST/GenericSignatureBuilder.h

@@ -426,10 +426,6 @@ private:
   template<typename F>
   void visitPotentialArchetypes(F f);
 
-  void markPotentialArchetypeRecursive(PotentialArchetype *pa,
-                                       ProtocolDecl *proto,
-                                       const RequirementSource *source);
-
 public:
   /// Construct a new generic signature builder.
   ///
@@ -1358,23 +1354,20 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// that share a name.
   llvm::MapVector<Identifier, StoredNestedType> NestedTypes;
 
-  /// \brief Recursively conforms to itself.
-  unsigned IsRecursive : 1;
-
   /// \brief Construct a new potential archetype for an unresolved
   /// associated type.
   PotentialArchetype(PotentialArchetype *parent, Identifier name);
 
   /// \brief Construct a new potential archetype for an associated type.
   PotentialArchetype(PotentialArchetype *parent, AssociatedTypeDecl *assocType)
-    : parentOrBuilder(parent), identifier(assocType), IsRecursive(false)
+    : parentOrBuilder(parent), identifier(assocType)
   {
     assert(parent != nullptr && "Not an associated type?");
   }
 
   /// \brief Construct a new potential archetype for a concrete declaration.
   PotentialArchetype(PotentialArchetype *parent, TypeDecl *concreteDecl)
-    : parentOrBuilder(parent), identifier(concreteDecl), IsRecursive(false)
+    : parentOrBuilder(parent), identifier(concreteDecl)
   {
     assert(parent != nullptr && "Not an associated type?");
   }
@@ -1382,8 +1375,7 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// \brief Construct a new potential archetype for a generic parameter.
   PotentialArchetype(GenericSignatureBuilder *builder,
                      GenericParamKey genericParam)
-    : parentOrBuilder(builder), identifier(genericParam),
+    : parentOrBuilder(builder), identifier(genericParam)
-      IsRecursive(false)
   {
   }
 
@@ -1626,9 +1618,6 @@ public:
     return Type();
   }
 
-  void setIsRecursive() { IsRecursive = true; }
-  bool isRecursive() const { return IsRecursive; }
-
   LLVM_ATTRIBUTE_DEPRECATED(
       void dump() const,
       "only for use within the debugger");

include/swift/Basic/LangOptions.h

@@ -210,9 +210,6 @@ namespace swift {
     /// Should we use \c ASTScope-based resolution for unqualified name lookup?
     bool EnableASTScopeLookup = false;
 
-    /// Enable SE-0157: Recursive Protocol Constraints.
-    bool EnableRecursiveConstraints = false;
-
     /// Whether to use the import as member inference system
     ///
     /// When importing a global, try to infer whether we can import it as a

include/swift/Option/FrontendOptions.td

@@ -109,9 +109,6 @@ def disable_target_os_checking :
 def enable_astscope_lookup : Flag<["-"], "enable-astscope-lookup">,
   HelpText<"Enable ASTScope-based unqualified name lookup">;
 
-def enable_recursive_constraints : Flag<["-"], "enable-recursive-constraints">,
-  HelpText<"Enable SE-0157: Recursive Protocol Constraints">;
-
 def print_clang_stats : Flag<["-"], "print-clang-stats">,
   HelpText<"Print Clang importer statistics">;
 

lib/AST/GenericSignatureBuilder.cpp

@@ -1945,9 +1945,7 @@ PotentialArchetype *PotentialArchetype::updateNestedTypeForConformance(
   // If we were asked for a complete, well-formed archetype, make sure we
   // process delayed requirements if anything changed.
   SWIFT_DEFER {
-    ASTContext &ctx = assocType ? assocType->getASTContext()
+    if (kind == ArchetypeResolutionKind::CompleteWellFormed)
-                                : concreteDecl->getASTContext();
-    if (ctx.LangOpts.EnableRecursiveConstraints)
       getBuilder()->processDelayedRequirements();
   };
 
@@ -3474,28 +3472,6 @@ ConstraintResult GenericSignatureBuilder::addSameTypeRequirementDirect(
   }
 }
 
-// Local function to mark the given associated type as recursive,
-// diagnosing it if this is the first such occurrence.
-void GenericSignatureBuilder::markPotentialArchetypeRecursive(
-    PotentialArchetype *pa, ProtocolDecl *proto, const RequirementSource *source) {
-  if (pa->isRecursive())
-    return;
-  pa->setIsRecursive();
-
-  pa->addConformance(proto, source, *this);
-  if (!pa->getParent())
-    return;
-
-  auto assocType = pa->getResolvedAssociatedType();
-  if (!assocType || assocType->isInvalid())
-    return;
-
-  Diags.diagnose(assocType->getLoc(), diag::recursive_requirement_reference);
-
-  // Silence downstream errors referencing this associated type.
-  assocType->setInvalid();
-}
-
 ConstraintResult GenericSignatureBuilder::addInheritedRequirements(
                              TypeDecl *decl,
                              UnresolvedType type,
@@ -3546,26 +3522,6 @@ ConstraintResult GenericSignatureBuilder::addInheritedRequirements(
                         getFloatingSource(typeRepr, /*forInferred=*/true));
     }
 
-    if (!decl->getASTContext().LangOpts.EnableRecursiveConstraints) {
-      // 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),

lib/Frontend/CompilerInvocation.cpp

@@ -1014,8 +1014,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   }
   
   Opts.EnableASTScopeLookup |= Args.hasArg(OPT_enable_astscope_lookup);
-  Opts.EnableRecursiveConstraints |=
-    Args.hasArg(OPT_enable_recursive_constraints);
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
   Opts.EnableConstraintPropagation |= Args.hasArg(OPT_propagate_constraints);
   Opts.IterativeTypeChecker |= Args.hasArg(OPT_iterative_type_checker);

test/Generics/same_type_constraints.swift

@@ -154,7 +154,7 @@ extension Dictionary {
 
 // rdar://problem/19245317
 protocol P {
-	associatedtype T: P // expected-error{{type may not reference itself as a requirement}}
+	associatedtype T: P
 }
 
 struct S<A: P> {

test/decl/protocol/recursive_requirement.swift

@@ -3,7 +3,7 @@
 // -----
 
 protocol Foo {
-  associatedtype Bar : Foo // expected-error{{type may not reference itself as a requirement}}
+  associatedtype Bar : Foo
 }
 
 struct Oroborous : Foo {
@@ -13,7 +13,7 @@ struct Oroborous : Foo {
 // -----
 
 protocol P {
- associatedtype A : P // expected-error{{type may not reference itself as a requirement}}
+ associatedtype A : P
 }
 
 struct X<T: P> {
@@ -26,7 +26,7 @@ func f<T : P>(_ z: T) {
 // -----
 
 protocol PP2 {
-  associatedtype A : P2 = Self // expected-error{{type may not reference itself as a requirement}}
+  associatedtype A : P2 = Self
 }
 
 protocol P2 : PP2 {
@@ -41,13 +41,13 @@ struct Y2 : P2 {
 }
 
 func f<T : P2>(_ z: T) {
- _ = X2<T.A>() // expected-error{{type 'T.A' does not conform to protocol 'P2'}}
+ _ = X2<T.A>()
 }
 
 // -----
 
 protocol P3 {
- associatedtype A: P4 = Self // expected-error{{type may not reference itself as a requirement}}
+ associatedtype A: P4 = Self
 }
 
 protocol P4 : P3 {}

test/decl/protocol/recursive_requirement_ok.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-recursive-constraints
+// RUN: %target-typecheck-verify-swift
 
 protocol P {
   associatedtype Assoc : P