SE-0309: Unlock existential types for all protocols

include/swift/AST/Decl.h

@@ -514,7 +514,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     IsComputingSemanticMembers : 1
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(ProtocolDecl, NominalTypeDecl, 1+1+1+1+1+1+1+1+1+1+1+8+16,
+  SWIFT_INLINE_BITFIELD_FULL(ProtocolDecl, NominalTypeDecl, 1+1+1+1+1+1+1+1+1+8+16,
     /// Whether the \c RequiresClass bit is valid.
     RequiresClassValid : 1,
 
@@ -527,12 +527,6 @@ class alignas(1 << DeclAlignInBits) Decl {
     /// Whether the existential of this protocol conforms to itself.
     ExistentialConformsToSelf : 1,
 
-    /// Whether the \c ExistentialTypeSupported bit is valid.
-    ExistentialTypeSupportedValid : 1,
-
-    /// Whether the existential of this protocol can be represented.
-    ExistentialTypeSupported : 1,
-
     /// True if the protocol has requirements that cannot be satisfied (e.g.
     /// because they could not be imported from Objective-C).
     HasMissingRequirements : 1,
@@ -4157,21 +4151,6 @@ class ProtocolDecl final : public NominalTypeDecl {
     Bits.ProtocolDecl.ExistentialConformsToSelf = result;
   }
 
-  /// Returns the cached result of \c existentialTypeSupported or \c None if it
-  /// hasn't yet been computed.
-  Optional<bool> getCachedExistentialTypeSupported() {
-    if (Bits.ProtocolDecl.ExistentialTypeSupportedValid)
-      return Bits.ProtocolDecl.ExistentialTypeSupported;
-
-    return None;
-  }
-
-  /// Caches the result of \c existentialTypeSupported
-  void setCachedExistentialTypeSupported(bool supported) {
-    Bits.ProtocolDecl.ExistentialTypeSupportedValid = true;
-    Bits.ProtocolDecl.ExistentialTypeSupported = supported;
-  }
-
   bool hasLazyRequirementSignature() const {
     return Bits.ProtocolDecl.HasLazyRequirementSignature;
   }
@@ -4181,7 +4160,6 @@ class ProtocolDecl final : public NominalTypeDecl {
   friend class RequirementSignatureRequest;
   friend class ProtocolRequiresClassRequest;
   friend class ExistentialConformsToSelfRequest;
-  friend class ExistentialTypeSupportedRequest;
   friend class InheritedProtocolsRequest;
 
 public:
@@ -4270,12 +4248,6 @@ class ProtocolDecl final : public NominalTypeDecl {
   /// contain 'Self' in 'parameter' or 'other' position.
   bool isAvailableInExistential(const ValueDecl *decl) const;
 
-  /// Determine whether we are allowed to refer to an existential type
-  /// conforming to this protocol. This is only permitted if the types of
-  /// all the members do not contain any associated types, and do not
-  /// contain 'Self' in 'parameter' or 'other' position.
-  bool existentialTypeSupported() const;
-
   /// Returns a list of protocol requirements that must be assessed to
   /// determine a concrete's conformance effect polymorphism kind.
   PolymorphicEffectRequirementList getPolymorphicEffectRequirements(

include/swift/AST/DiagnosticsSema.def

@@ -920,9 +920,6 @@ NOTE(object_literal_resolve_import,none,
 
 ERROR(use_local_before_declaration,none,
       "use of local variable %0 before its declaration", (DeclNameRef))
-ERROR(unsupported_existential_type,none,
-      "protocol %0 can only be used as a generic constraint because it has "
-      "Self or associated type requirements", (Identifier))
 
 ERROR(decl_does_not_exist_in_module,none,
       "%select{%error|type|struct|class|enum|protocol|variable|function}0 "

include/swift/AST/EducationalNotes.def

@@ -21,8 +21,8 @@
 
 // EDUCATIONAL_NOTES(DIAG_ID, EDUCATIONAL_NOTE_FILENAMES...)
 
-EDUCATIONAL_NOTES(unsupported_existential_type,
-                  "associated-type-requirements.md")
+EDUCATIONAL_NOTES(could_not_use_member_on_existential,
+                  "existential-member-access-limitations.md")
 
 EDUCATIONAL_NOTES(cannot_pass_type_to_non_ephemeral, "temporary-pointers.md")
 EDUCATIONAL_NOTES(cannot_pass_type_to_non_ephemeral_warning,

include/swift/AST/TypeCheckRequests.h

@@ -287,32 +287,6 @@ class ExistentialConformsToSelfRequest :
   void cacheResult(bool value) const;
 };
 
-/// Determine whether we are allowed to refer to an existential type conforming
-/// to this protocol.
-class ExistentialTypeSupportedRequest :
-    public SimpleRequest<ExistentialTypeSupportedRequest,
-                         bool(ProtocolDecl *),
-                         RequestFlags::SeparatelyCached> {
-public:
-  using SimpleRequest::SimpleRequest;
-
-private:
-  friend SimpleRequest;
-
-  // Evaluation.
-  bool evaluate(Evaluator &evaluator, ProtocolDecl *decl) const;
-
-public:
-  // Cycle handling.
-  void diagnoseCycle(DiagnosticEngine &diags) const;
-  void noteCycleStep(DiagnosticEngine &diags) const;
-
-  // Separate caching.
-  bool isCached() const { return true; }
-  Optional<bool> getCachedResult() const;
-  void cacheResult(bool value) const;
-};
-
 class PolymorphicEffectRequirementsRequest :
     public SimpleRequest<PolymorphicEffectRequirementsRequest,
                          PolymorphicEffectRequirementList(EffectKind, ProtocolDecl *),

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -87,8 +87,6 @@ SWIFT_REQUEST(TypeChecker, EnumRawTypeRequest,
               Type(EnumDecl *), Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ExistentialConformsToSelfRequest,
               bool(ProtocolDecl *), SeparatelyCached, NoLocationInfo)
-SWIFT_REQUEST(TypeChecker, ExistentialTypeSupportedRequest,
-              bool(ProtocolDecl *), SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ExtendedTypeRequest, Type(ExtensionDecl *), Cached,
               NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ResultBuilderTypeRequest, Type(ValueDecl *),

lib/AST/Decl.cpp

@@ -5020,24 +5020,31 @@ findProtocolSelfReferences(const ProtocolDecl *proto, Type type,
     return findProtocolSelfReferences(proto, selfType->getSelfType(), position);
   }
 
-  // Most bound generic types are invariant.
-  if (auto *const bgt = type->getAs<BoundGenericType>()) {
+  if (auto *const nominal = type->getAs<NominalOrBoundGenericNominalType>()) {
     auto info = SelfReferenceInfo();
 
-    if (bgt->isArray()) {
-      // Swift.Array preserves variance in its Value type.
-      info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().front(),
-                                         position);
-    } else if (bgt->isDictionary()) {
-      // Swift.Dictionary preserves variance in its Element type.
-      info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().front(),
-                                         SelfReferencePosition::Invariant);
-      info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().back(),
-                                         position);
-    } else {
-      for (auto paramType : bgt->getGenericArgs()) {
-        info |= findProtocolSelfReferences(proto, paramType,
+    // Don't forget to look in the parent.
+    if (const auto parent = nominal->getParent()) {
+      info |= findProtocolSelfReferences(proto, parent, position);
+    }
+
+    // Most bound generic types are invariant.
+    if (auto *const bgt = type->getAs<BoundGenericType>()) {
+      if (bgt->isArray()) {
+        // Swift.Array preserves variance in its Value type.
+        info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().front(),
+                                           position);
+      } else if (bgt->isDictionary()) {
+        // Swift.Dictionary preserves variance in its Element type.
+        info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().front(),
                                            SelfReferencePosition::Invariant);
+        info |= findProtocolSelfReferences(proto, bgt->getGenericArgs().back(),
+                                           position);
+      } else {
+        for (auto paramType : bgt->getGenericArgs()) {
+          info |= findProtocolSelfReferences(proto, paramType,
+                                             SelfReferencePosition::Invariant);
+        }
       }
     }
 
@@ -5049,6 +5056,16 @@ findProtocolSelfReferences(const ProtocolDecl *proto, Type type,
   if (type->is<OpaqueTypeArchetypeType>())
     return SelfReferenceInfo::forSelfRef(SelfReferencePosition::Invariant);
 
+  // Protocol compositions preserve variance.
+  if (auto *comp = type->getAs<ProtocolCompositionType>()) {
+    // 'Self' may be referenced only in a superclass component.
+    if (const auto superclass = comp->getSuperclass()) {
+      return findProtocolSelfReferences(proto, superclass, position);
+    }
+
+    return SelfReferenceInfo();
+  }
+
   // A direct reference to 'Self'.
   if (proto->getSelfInterfaceType()->isEqual(type))
     return SelfReferenceInfo::forSelfRef(position);
@@ -5148,11 +5165,6 @@ bool ProtocolDecl::isAvailableInExistential(const ValueDecl *decl) const {
   return true;
 }
 
-bool ProtocolDecl::existentialTypeSupported() const {
-  return evaluateOrDefault(getASTContext().evaluator,
-    ExistentialTypeSupportedRequest{const_cast<ProtocolDecl *>(this)}, true);
-}
-
 StringRef ProtocolDecl::getObjCRuntimeName(
                           llvm::SmallVectorImpl<char> &buffer) const {
   // If there is an 'objc' attribute with a name, use that name.

lib/AST/TypeCheckRequests.cpp

@@ -253,31 +253,6 @@ void ExistentialConformsToSelfRequest::cacheResult(bool value) const {
   decl->setCachedExistentialConformsToSelf(value);
 }
 
-//----------------------------------------------------------------------------//
-// existentialTypeSupported computation.
-//----------------------------------------------------------------------------//
-
-void ExistentialTypeSupportedRequest::diagnoseCycle(DiagnosticEngine &diags) const {
-  auto decl = std::get<0>(getStorage());
-  diags.diagnose(decl, diag::circular_protocol_def, decl->getName());
-}
-
-void ExistentialTypeSupportedRequest::noteCycleStep(DiagnosticEngine &diags) const {
-  auto requirement = std::get<0>(getStorage());
-  diags.diagnose(requirement, diag::kind_declname_declared_here,
-                 DescriptiveDeclKind::Protocol, requirement->getName());
-}
-
-Optional<bool> ExistentialTypeSupportedRequest::getCachedResult() const {
-  auto decl = std::get<0>(getStorage());
-  return decl->getCachedExistentialTypeSupported();
-}
-
-void ExistentialTypeSupportedRequest::cacheResult(bool value) const {
-  auto decl = std::get<0>(getStorage());
-  decl->setCachedExistentialTypeSupported(value);
-}
-
 //----------------------------------------------------------------------------//
 // isFinal computation.
 //----------------------------------------------------------------------------//

lib/Sema/CSSimplify.cpp

@@ -7302,7 +7302,7 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     // Dig out the instance type and figure out what members of the instance type
     // we are going to see.
     auto baseTy = candidate.getBaseType();
-    auto baseObjTy = baseTy->getRValueType();
+    const auto baseObjTy = baseTy->getRValueType();
 
     bool hasInstanceMembers = false;
     bool hasInstanceMethods = false;
@@ -7349,18 +7349,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       hasInstanceMethods = true;
     }
 
-    // If our base is an existential type, we can't make use of any
-    // member whose signature involves associated types.
-    if (instanceTy->isExistentialType()) {
-      if (auto *proto = decl->getDeclContext()->getSelfProtocolDecl()) {
-        if (!proto->isAvailableInExistential(decl)) {
-          result.addUnviable(candidate,
-                             MemberLookupResult::UR_UnavailableInExistential);
-          return;
-        }
-      }
-    }
-
     // If the invocation's argument expression has a favored type,
     // use that information to determine whether a specific overload for
     // the candidate should be favored.
@@ -7380,6 +7368,20 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       }
     }
 
+    const auto isUnsupportedExistentialMemberAccess = [&] {
+      // If our base is an existential type, we can't make use of any
+      // member whose signature involves associated types.
+      if (instanceTy->isExistentialType()) {
+        if (auto *proto = decl->getDeclContext()->getSelfProtocolDecl()) {
+          if (!proto->isAvailableInExistential(decl)) {
+            return true;
+          }
+        }
+      }
+
+      return false;
+    };
+
     // See if we have an instance method, instance member or static method,
     // and check if it can be accessed on our base type.
 
@@ -7393,20 +7395,35 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
                           ? candidate
                           : OverloadChoice(instanceTy, decl,
                                            FunctionRefKind::SingleApply);
-        // If this is an instance member referenced from metatype
-        // let's add unviable result to the set because it could be
-        // either curried reference or an  invalid call.
-        //
-        // New candidate shouldn't affect performance because such
-        // choice would only be attempted when solver is in diagnostic mode.
-        result.addUnviable(choice, MemberLookupResult::UR_InstanceMemberOnType);
-
-        bool invalidMethodRef = isa<FuncDecl>(decl) && !hasInstanceMethods;
-        bool invalidMemberRef = !isa<FuncDecl>(decl) && !hasInstanceMembers;
-        // If this is definitely an invalid way to reference a method or member
-        // on the metatype, let's stop here.
-        if (invalidMethodRef || invalidMemberRef)
+
+        const bool invalidMethodRef = isa<FuncDecl>(decl) && !hasInstanceMethods;
+        const bool invalidMemberRef = !isa<FuncDecl>(decl) && !hasInstanceMembers;
+
+        if (invalidMethodRef || invalidMemberRef) {
+          // If this is definitely an invalid way to reference a method or member
+          // on the metatype, let's stop here.
+          result.addUnviable(choice,
+                             MemberLookupResult::UR_InstanceMemberOnType);
+          return;
+        } else if (isUnsupportedExistentialMemberAccess()) {
+          // If the member reference itself is legal, but it turns out to be an
+          // unsupported existential member access, do not make further
+          // assumptions about the correctness of a potential call -- let
+          // the unsupported member access error prevail.
+          result.addUnviable(candidate,
+                             MemberLookupResult::UR_UnavailableInExistential);
           return;
+        } else {
+          // Otherwise, still add an unviable result to the set, because it
+          // could be an invalid call that was supposed to be performed on an
+          // instance of the type.
+          //
+          // New candidate shouldn't affect performance because such
+          // choice would only be attempted when solver is in diagnostic mode.
+          result.addUnviable(choice,
+                             MemberLookupResult::UR_InstanceMemberOnType);
+
+        }
       }
 
     // If the underlying type of a typealias is fully concrete, it is legal
@@ -7457,6 +7474,12 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       }
     }
 
+    if (isUnsupportedExistentialMemberAccess()) {
+      result.addUnviable(candidate,
+                         MemberLookupResult::UR_UnavailableInExistential);
+      return;
+    }
+
     // If we have an rvalue base, make sure that the result isn't 'mutating'
     // (only valid on lvalues).
     if (!baseTy->is<AnyMetatypeType>() &&

lib/Sema/MiscDiagnostics.cpp

@@ -3342,8 +3342,6 @@ static void checkSwitch(ASTContext &ctx, const SwitchStmt *stmt) {
   // We want to warn about "case .Foo, .Bar where 1 != 100:" since the where
   // clause only applies to the second case, and this is surprising.
   for (auto cs : stmt->getCases()) {
-    TypeChecker::checkUnsupportedProtocolType(ctx, cs);
-
     // The case statement can have multiple case items, each can have a where.
     // If we find a "where", and there is a preceding item without a where, and
     // if they are on the same source line, then warn.
@@ -4760,8 +4758,6 @@ void swift::performSyntacticExprDiagnostics(const Expr *E,
 void swift::performStmtDiagnostics(const Stmt *S, DeclContext *DC) {
   auto &ctx = DC->getASTContext();
 
-  TypeChecker::checkUnsupportedProtocolType(ctx, const_cast<Stmt *>(S));
-
   if (auto switchStmt = dyn_cast<SwitchStmt>(S))
     checkSwitch(ctx, switchStmt);
 

lib/Sema/TypeCheckDecl.cpp

@@ -672,34 +672,6 @@ ExistentialConformsToSelfRequest::evaluate(Evaluator &evaluator,
   return true;
 }
 
-bool
-ExistentialTypeSupportedRequest::evaluate(Evaluator &evaluator,
-                                          ProtocolDecl *decl) const {
-  // ObjC protocols can always be existential.
-  if (decl->isObjC())
-    return true;
-
-  for (auto member : decl->getMembers()) {
-    // Existential types cannot be used if the protocol has an associated type.
-    if (isa<AssociatedTypeDecl>(member))
-      return false;
-
-    // For value members, look at their type signatures.
-    if (auto valueMember = dyn_cast<ValueDecl>(member)) {
-      if (!decl->isAvailableInExistential(valueMember))
-        return false;
-    }
-  }
-
-  // Check whether all of the inherited protocols support existential types.
-  for (auto proto : decl->getInheritedProtocols()) {
-    if (!proto->existentialTypeSupported())
-      return false;
-  }
-
-  return true;
-}
-
 bool
 IsFinalRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
   if (isa<ClassDecl>(decl))