[SE-0335] Introduce existential any

include/swift/AST/Decl.h

@@ -520,7 +520,7 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
     IsComputingSemanticMembers : 1
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(ProtocolDecl, NominalTypeDecl, 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+1+1+8+16,
     /// Whether the \c RequiresClass bit is valid.
     RequiresClassValid : 1,
 
@@ -533,6 +533,12 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
     /// Whether the existential of this protocol conforms to itself.
     ExistentialConformsToSelf : 1,
 
+    /// Whether the \c ExistentialRequiresAny bit is valid.
+    ExistentialRequiresAnyValid : 1,
+
+    /// Whether the existential of this protocol must be spelled with \c any.
+    ExistentialRequiresAny : 1,
+
     /// True if the protocol has requirements that cannot be satisfied (e.g.
     /// because they could not be imported from Objective-C).
     HasMissingRequirements : 1,
@@ -4244,6 +4250,21 @@ class ProtocolDecl final : public NominalTypeDecl {
     Bits.ProtocolDecl.ExistentialConformsToSelf = result;
   }
 
+  /// Returns the cached result of \c existentialRequiresAny or \c None if it
+  /// hasn't yet been computed.
+  Optional<bool> getCachedExistentialRequiresAny() {
+    if (Bits.ProtocolDecl.ExistentialRequiresAnyValid)
+      return Bits.ProtocolDecl.ExistentialRequiresAny;
+
+    return None;
+  }
+
+  /// Caches the result of \c existentialRequiresAny
+  void setCachedExistentialRequiresAny(bool requiresAny) {
+    Bits.ProtocolDecl.ExistentialRequiresAnyValid = true;
+    Bits.ProtocolDecl.ExistentialRequiresAny = requiresAny;
+  }
+
   bool hasLazyRequirementSignature() const {
     return Bits.ProtocolDecl.HasLazyRequirementSignature;
   }
@@ -4257,6 +4278,7 @@ class ProtocolDecl final : public NominalTypeDecl {
   friend class RequirementSignatureRequestRQM;
   friend class ProtocolRequiresClassRequest;
   friend class ExistentialConformsToSelfRequest;
+  friend class ExistentialRequiresAnyRequest;
   friend class InheritedProtocolsRequest;
 
 public:
@@ -4345,6 +4367,11 @@ class ProtocolDecl final : public NominalTypeDecl {
   /// contain 'Self' in 'parameter' or 'other' position.
   bool isAvailableInExistential(const ValueDecl *decl) const;
 
+  /// Determine whether an existential type must be explicitly prefixed
+  /// with \c any. \c any is required if any of the members contain
+  /// an associated type, or if \c Self appears in non-covariant position.
+  bool existentialRequiresAny() 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/DiagnosticsParse.def

@@ -873,7 +873,8 @@ ERROR(sil_function_subs_without_generics,none,
 
 // Opaque types
 ERROR(opaque_mid_composition,none,
-      "'some' should appear at the beginning of a composition", ())
+      "'%0' should appear at the beginning of a composition",
+      (StringRef))
 
 //------------------------------------------------------------------------------
 // MARK: Layout constraint diagnostics

include/swift/AST/DiagnosticsSema.def

@@ -4618,6 +4618,19 @@ ERROR(unchecked_not_inheritance_clause,none,
 ERROR(unchecked_not_existential,none,
       "'unchecked' attribute cannot apply to non-protocol type %0", (Type))
 
+WARNING(unnecessary_any,none,
+         "'any' is redundant on type %0", (Type))
+ERROR(any_not_existential,none,
+       "'any' has no effect on %select{concrete type|type parameter}0 %1",
+       (bool, Type))
+ERROR(existential_requires_any,none,
+      "protocol %0 as a type must be explicitly marked as 'any'",
+      (Identifier))
+ERROR(explicit_existential_not_supported,none,
+       "explicit 'any' not supported; use frontend flag "
+       "-enable-explicit-existential-types to enable this feature",
+       ())
+
 ERROR(nonisolated_let,none,
       "'nonisolated' is meaningless on 'let' declarations because "
       "they are immutable",

include/swift/AST/TypeCheckRequests.h

@@ -287,6 +287,32 @@ class ExistentialConformsToSelfRequest :
   void cacheResult(bool value) const;
 };
 
+/// Determine whether an existential type conforming to this protocol
+/// requires the \c any syntax.
+class ExistentialRequiresAnyRequest :
+    public SimpleRequest<ExistentialRequiresAnyRequest,
+                         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

@@ -92,6 +92,8 @@ SWIFT_REQUEST(TypeChecker, EnumRawTypeRequest,
               Type(EnumDecl *), Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ExistentialConformsToSelfRequest,
               bool(ProtocolDecl *), SeparatelyCached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, ExistentialRequiresAnyRequest,
+              bool(ProtocolDecl *), SeparatelyCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ExtendedTypeRequest, Type(ExtensionDecl *), Cached,
               NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ResultBuilderTypeRequest, Type(ValueDecl *),

include/swift/AST/TypeDifferenceVisitor.h

@@ -328,6 +328,12 @@ class CanTypeDifferenceVisitor : public CanTypePairVisitor<Impl, bool> {
                                type1->getMembers(), type2->getMembers());
   }
 
+  bool visitExistentialType(CanExistentialType type1,
+                            CanExistentialType type2) {
+    return asImpl().visit(type1.getConstraintType(),
+                          type2.getConstraintType());
+  }
+
   bool visitLValueType(CanLValueType type1, CanLValueType type2) {
     return asImpl().visit(type1.getObjectType(), type2.getObjectType());
   }

include/swift/AST/TypeMatcher.h

@@ -269,6 +269,20 @@ class TypeMatcher {
     TRIVIAL_CASE(SILBoxType)
     TRIVIAL_CASE(ProtocolCompositionType)
 
+    bool visitExistentialType(CanExistentialType firstExistential,
+                              Type secondType,
+                              Type sugaredFirstType) {
+      if (auto secondExistential = secondType->getAs<ExistentialType>()) {
+        return this->visit(firstExistential.getConstraintType(),
+                           secondExistential->getConstraintType(),
+                           sugaredFirstType->castTo<ExistentialType>()
+                               ->getConstraintType());
+      }
+
+      return mismatch(firstExistential.getPointer(), secondType,
+                      sugaredFirstType);
+    }
+
     bool visitLValueType(CanLValueType firstLValue, Type secondType,
                          Type sugaredFirstType) {
       if (auto secondLValue = secondType->getAs<LValueType>()) {

include/swift/AST/TypeNodes.def

@@ -162,6 +162,7 @@ ARTIFICIAL_TYPE(SILBlockStorage, Type)
 ARTIFICIAL_TYPE(SILBox, Type)
 ARTIFICIAL_TYPE(SILToken, Type)
 TYPE(ProtocolComposition, Type)
+TYPE(Existential, Type)
 TYPE(LValue, Type)
 TYPE(InOut, Type)
 UNCHECKED_TYPE(TypeVariable, Type)

include/swift/AST/TypeRepr.h

@@ -1158,6 +1158,35 @@ class NamedOpaqueReturnTypeRepr : public TypeRepr {
   friend class TypeRepr;
 };
 
+/// A TypeRepr for an existential type spelled with \c any
+///
+/// Can appear anywhere a normal existential type would. This is
+/// purely a more explicit spelling for existential types.
+class ExistentialTypeRepr: public TypeRepr {
+  TypeRepr *Constraint;
+  SourceLoc AnyLoc;
+
+public:
+  ExistentialTypeRepr(SourceLoc anyLoc, TypeRepr *constraint)
+    : TypeRepr(TypeReprKind::Existential), Constraint(constraint),
+      AnyLoc(anyLoc) {}
+
+  TypeRepr *getConstraint() const { return Constraint; }
+  SourceLoc getAnyLoc() const { return AnyLoc; }
+
+  static bool classof(const TypeRepr *T) {
+    return T->getKind() == TypeReprKind::Existential;
+  }
+  static bool classof(const ExistentialTypeRepr *T) { return true; }
+
+private:
+  SourceLoc getStartLocImpl() const { return AnyLoc; }
+  SourceLoc getEndLocImpl() const { return Constraint->getEndLoc(); }
+  SourceLoc getLocImpl() const { return AnyLoc; }
+  void printImpl(ASTPrinter &Printer, const PrintOptions &Opts) const;
+  friend class TypeRepr;
+};
+
 /// TypeRepr for a user-specified placeholder (essentially, a user-facing
 /// representation of an anonymous type variable.
 ///
@@ -1285,6 +1314,7 @@ inline bool TypeRepr::isSimple() const {
   case TypeReprKind::Composition:
   case TypeReprKind::OpaqueReturn:
   case TypeReprKind::NamedOpaqueReturn:
+  case TypeReprKind::Existential:
     return false;
   case TypeReprKind::SimpleIdent:
   case TypeReprKind::GenericIdent:

include/swift/AST/TypeReprNodes.def

@@ -55,6 +55,7 @@ TYPEREPR(Metatype, TypeRepr)
 TYPEREPR(Protocol, TypeRepr)
 TYPEREPR(OpaqueReturn, TypeRepr)
 TYPEREPR(NamedOpaqueReturn, TypeRepr)
+TYPEREPR(Existential, TypeRepr)
 TYPEREPR(Placeholder, TypeRepr)
 ABSTRACT_TYPEREPR(Specifier, TypeRepr)
   TYPEREPR(InOut, SpecifierTypeRepr)

include/swift/AST/Types.h

@@ -5202,6 +5202,42 @@ class ProtocolCompositionType final : public TypeBase,
 BEGIN_CAN_TYPE_WRAPPER(ProtocolCompositionType, Type)
 END_CAN_TYPE_WRAPPER(ProtocolCompositionType, Type)
 
+/// An existential type, spelled with \c any .
+///
+/// In Swift 5 mode, a plain protocol name in type
+/// context is an implicit existential type.
+class ExistentialType final : public TypeBase {
+  Type ConstraintType;
+
+  ExistentialType(Type constraintType,
+                  const ASTContext *canonicalContext,
+                  RecursiveTypeProperties properties)
+      : TypeBase(TypeKind::Existential, canonicalContext, properties),
+        ConstraintType(constraintType) {}
+
+public:
+  static ExistentialType *get(Type constraint);
+
+  Type getConstraintType() const { return ConstraintType; }
+
+  bool requiresClass() const {
+    if (auto protocol = ConstraintType->getAs<ProtocolType>())
+      return protocol->requiresClass();
+
+    if (auto composition = ConstraintType->getAs<ProtocolCompositionType>())
+      return composition->requiresClass();
+
+    return false;
+  }
+
+  static bool classof(const TypeBase *type) {
+    return type->getKind() == TypeKind::Existential;
+  }
+};
+BEGIN_CAN_TYPE_WRAPPER(ExistentialType, Type)
+  PROXY_CAN_TYPE_SIMPLE_GETTER(getConstraintType)
+END_CAN_TYPE_WRAPPER(ExistentialType, Type)
+
 /// LValueType - An l-value is a handle to a physical object.  The
 /// type of that object uniquely determines the type of an l-value
 /// for it.
@@ -6157,7 +6193,9 @@ inline bool TypeBase::isAnyExistentialType() {
 }
 
 inline bool CanType::isExistentialTypeImpl(CanType type) {
-  return isa<ProtocolType>(type) || isa<ProtocolCompositionType>(type);
+  return (isa<ProtocolType>(type) ||
+          isa<ProtocolCompositionType>(type) ||
+          isa<ExistentialType>(type));
 }
 
 inline bool CanType::isAnyExistentialTypeImpl(CanType type) {
@@ -6170,6 +6208,8 @@ inline bool TypeBase::isClassExistentialType() {
     return pt->requiresClass();
   if (auto pct = dyn_cast<ProtocolCompositionType>(T))
     return pct->requiresClass();
+  if (auto existential = dyn_cast<ExistentialType>(T))
+    return existential->requiresClass();
   return false;
 }
 
@@ -6428,6 +6468,9 @@ inline bool TypeBase::hasSimpleTypeRepr() const {
   case TypeKind::ExistentialMetatype:
     return !cast<const AnyMetatypeType>(this)->hasRepresentation();
 
+  case TypeKind::Existential:
+    return false;
+
   case TypeKind::NestedArchetype:
     return cast<NestedArchetypeType>(this)->getParent()->hasSimpleTypeRepr();
 

include/swift/Basic/LangOptions.h

@@ -310,6 +310,10 @@ namespace swift {
     /// `func f() -> <T> T`.
     bool EnableExperimentalNamedOpaqueTypes = false;
 
+    /// Enable support for explicit existential types via the \c any
+    /// keyword.
+    bool EnableExplicitExistentialTypes = false;
+
     /// Enable experimental flow-sensitive concurrent captures.
     bool EnableExperimentalFlowSensitiveConcurrentCaptures = false;
 

include/swift/Option/FrontendOptions.td

@@ -476,6 +476,10 @@ def enable_experimental_structural_opaque_types :
   Flag<["-"], "enable-experimental-structural-opaque-types">,
   HelpText<"Enable experimental support for structural opaque result types">;
 
+def enable_explicit_existential_types :
+  Flag<["-"], "enable-explicit-existential-types">,
+  HelpText<"Enable experimental support for explicit existential types">;
+
 def enable_deserialization_recovery :
   Flag<["-"], "enable-deserialization-recovery">,
   HelpText<"Attempt to recover from missing xrefs (etc) in swiftmodules">;

lib/AST/ASTContext.cpp

@@ -408,6 +408,7 @@ struct ASTContext::Implementation {
     llvm::DenseMap<std::pair<StructDecl*, Type>, StructType*> StructTypes;
     llvm::DenseMap<std::pair<ClassDecl*, Type>, ClassType*> ClassTypes;
     llvm::DenseMap<std::pair<ProtocolDecl*, Type>, ProtocolType*> ProtocolTypes;
+    llvm::DenseMap<Type, ExistentialType *> ExistentialTypes;
     llvm::FoldingSet<UnboundGenericType> UnboundGenericTypes;
     llvm::FoldingSet<BoundGenericType> BoundGenericTypes;
     llvm::FoldingSet<ProtocolCompositionType> ProtocolCompositionTypes;
@@ -4098,6 +4099,21 @@ ProtocolType::ProtocolType(ProtocolDecl *TheDecl, Type Parent,
                            RecursiveTypeProperties properties)
   : NominalType(TypeKind::Protocol, &Ctx, TheDecl, Parent, properties) { }
 
+ExistentialType *ExistentialType::get(Type constraint) {
+  auto properties = constraint->getRecursiveProperties();
+  auto arena = getArena(properties);
+
+  auto &C = constraint->getASTContext();
+  auto &entry = C.getImpl().getArena(arena).ExistentialTypes[constraint];
+  if (entry)
+    return entry;
+
+  const ASTContext *canonicalContext = constraint->isCanonical() ? &C : nullptr;
+  return entry = new (C, arena) ExistentialType(constraint,
+                                                canonicalContext,
+                                                properties);
+}
+
 LValueType *LValueType::get(Type objectTy) {
   assert(!objectTy->is<LValueType>() && !objectTy->is<InOutType>() &&
          "cannot have 'inout' or @lvalue wrapped inside an @lvalue");

lib/AST/ASTDemangler.cpp

@@ -589,7 +589,12 @@ Type ASTBuilder::createProtocolCompositionType(
     members.push_back(protocol->getDeclaredInterfaceType());
   if (superclass && superclass->getClassOrBoundGenericClass())
     members.push_back(superclass);
-  return ProtocolCompositionType::get(Ctx, members, isClassBound);
+  Type composition = ProtocolCompositionType::get(Ctx, members, isClassBound);
+  if (Ctx.LangOpts.EnableExplicitExistentialTypes &&
+      !(composition->isAny() || composition->isAnyObject())) {
+    composition = ExistentialType::get(composition);
+  }
+  return composition;
 }
 
 static MetatypeRepresentation
@@ -607,6 +612,8 @@ getMetatypeRepresentation(ImplMetatypeRepresentation repr) {
 
 Type ASTBuilder::createExistentialMetatypeType(Type instance,
                           Optional<Demangle::ImplMetatypeRepresentation> repr) {
+  if (auto existential = instance->getAs<ExistentialType>())
+    instance = existential->getConstraintType();
   if (!instance->isAnyExistentialType())
     return Type();
   if (!repr)