Rough sketch of AST/Sema support for tuple conformances

include/swift/AST/ASTContext.h

@@ -66,6 +66,7 @@ namespace swift {
   enum class Associativity : unsigned char;
   class AvailabilityContext;
   class BoundGenericType;
+  class BuiltinTupleDecl;
   class ClangModuleLoader;
   class ClangNode;
   class ClangTypeConverter;
@@ -953,7 +954,7 @@ class ASTContext final {
   const CanType TheIEEE80Type;            /// 80-bit IEEE floating point
   const CanType TheIEEE128Type;           /// 128-bit IEEE floating point
   const CanType ThePPC128Type;            /// 128-bit PowerPC 2xDouble
-  
+
   /// Adds a search path to SearchPathOpts, unless it is already present.
   ///
   /// Does any proper bookkeeping to keep all module loaders up to date as well.
@@ -1421,6 +1422,13 @@ class ASTContext final {
   /// invocation decoder of the given distributed actor.
   FuncDecl *getDistributedActorArgumentDecodingMethod(NominalTypeDecl *);
 
+  /// The special Builtin.TheTupleType, which parents tuple extensions and
+  /// conformances.
+  BuiltinTupleDecl *getBuiltinTupleDecl();
+
+  /// The declared interface type of Builtin.TheTupleType.
+  BuiltinTupleType *getBuiltinTupleType();
+
 private:
   friend Decl;
 

include/swift/AST/Attr.def

@@ -58,7 +58,6 @@ TYPE_ATTR(Sendable)
 TYPE_ATTR(unchecked)
 TYPE_ATTR(_typeSequence)
 TYPE_ATTR(_local)
-TYPE_ATTR(tuple)
 
 // SIL-specific attributes
 TYPE_ATTR(block_storage)

include/swift/AST/Attr.h

@@ -2542,9 +2542,6 @@ class TypeAttributes {
   };
   Optional<OpaqueReturnTypeRef> OpaqueReturnTypeOf;
 
-  // Force construction of a one-element tuple type.
-  bool IsTuple = false;
-
   TypeAttributes() {}
 
   bool isValid() const { return AtLoc.isValid(); }

include/swift/AST/Decl.h

@@ -4755,6 +4755,48 @@ class ProtocolDecl final : public NominalTypeDecl {
   }
 };
 
+/// This is the special singleton Builtin.TheTupleType. It is not directly
+/// visible in the source language, but we use it to attach extensions
+/// and conformances for tuple types.
+///
+/// - The declared interface type is the special TheTupleType singleton.
+/// - The generic parameter list has one pack generic parameter, <Elements...>
+/// - The generic signature has no requirements, <Elements...>
+/// - The self interface type is the tuple type containing a single pack
+///   expansion, (Elements...).
+class BuiltinTupleDecl final : public NominalTypeDecl {
+  TupleType *TupleSelfType = nullptr;
+
+public:
+  BuiltinTupleDecl(Identifier Name, DeclContext *Parent);
+
+  SourceRange getSourceRange() const {
+    return SourceRange();
+  }
+
+  TupleType *getTupleSelfType() const;
+
+  // Implement isa/cast/dyncast/etc.
+  static bool classof(const Decl *D) {
+    return D->getKind() == DeclKind::BuiltinTuple;
+  }
+  static bool classof(const GenericTypeDecl *D) {
+    return D->getKind() == DeclKind::BuiltinTuple;
+  }
+  static bool classof(const NominalTypeDecl *D) {
+    return D->getKind() == DeclKind::BuiltinTuple;
+  }
+  static bool classof(const DeclContext *C) {
+    if (auto D = C->getAsDecl())
+      return classof(D);
+    return false;
+  }
+  static bool classof(const IterableDeclContext *C) {
+    auto NTD = dyn_cast<NominalTypeDecl>(C);
+    return NTD && classof(NTD);
+  }
+};
+
 /// AbstractStorageDecl - This is the common superclass for VarDecl and
 /// SubscriptDecl, representing potentially settable memory locations.
 class AbstractStorageDecl : public ValueDecl {

include/swift/AST/DeclNodes.def

@@ -151,7 +151,8 @@ ABSTRACT_DECL(Value, Decl)
         NOMINAL_TYPE_DECL(Struct, NominalTypeDecl)
         NOMINAL_TYPE_DECL(Class, NominalTypeDecl)
         NOMINAL_TYPE_DECL(Protocol, NominalTypeDecl)
-        DECL_RANGE(NominalType, Enum, Protocol)
+        NOMINAL_TYPE_DECL(BuiltinTuple, NominalTypeDecl)
+        DECL_RANGE(NominalType, Enum, BuiltinTuple)
       GENERIC_VALUE_DECL(OpaqueType, GenericTypeDecl)
       GENERIC_VALUE_DECL(TypeAlias, GenericTypeDecl)
       DECL_RANGE(GenericType, Enum, TypeAlias)

include/swift/AST/DiagnosticsSema.def

@@ -5179,8 +5179,6 @@ ERROR(opened_bad_interface_type,none,
       "@opened interface type %0 is not a type parameter", (Type))
 ERROR(sil_function_input_label,PointsToFirstBadToken,
       "SIL function types cannot have labeled inputs", ())
-ERROR(sil_function_output_label,PointsToFirstBadToken,
-      "SIL function types cannot have labeled results", ())
 ERROR(sil_non_coro_yields,PointsToFirstBadToken,
       "non-coroutine SIL function types cannot have @yield results", ())
 ERROR(sil_function_repeat_convention,PointsToFirstBadToken,

include/swift/AST/KnownIdentifiers.def

@@ -75,6 +75,7 @@ IDENTIFIER_WITH_NAME(PatternMatchVar, "$match")
 IDENTIFIER(dynamicallyCall)
 IDENTIFIER(dynamicMember)
 IDENTIFIER(Element)
+IDENTIFIER(Elements)
 IDENTIFIER_(enclosingInstance)
 IDENTIFIER(Encodable)
 IDENTIFIER(encode)
@@ -313,6 +314,9 @@ IDENTIFIER_WITH_NAME(TypeWrapperProperty, "$_storage")
 IDENTIFIER(storageKeyPath)
 IDENTIFIER(memberwise)
 
+// The singleton instance of TupleTypeDecl in the Builtin module
+IDENTIFIER(TheTupleType)
+
 #undef IDENTIFIER
 #undef IDENTIFIER_
 #undef IDENTIFIER_WITH_NAME

include/swift/AST/TypeNodes.def

@@ -128,7 +128,8 @@ ABSTRACT_TYPE(AnyGeneric, Type)
       TYPE(Struct, NominalType)
       TYPE(Class, NominalType)
       TYPE(Protocol, NominalType)
-      TYPE_RANGE(Nominal, Enum, Protocol)
+      TYPE(BuiltinTuple, NominalType)
+      TYPE_RANGE(Nominal, Enum, BuiltinTuple)
     ABSTRACT_TYPE(BoundGeneric, Type)
       TYPE(BoundGenericClass, BoundGenericType)
       TYPE(BoundGenericEnum, BoundGenericType)

include/swift/AST/Types.h

@@ -58,6 +58,7 @@ class ArgumentList;
 class AssociatedTypeDecl;
 class ASTContext;
 enum BufferPointerTypeKind : unsigned;
+class BuiltinTupleDecl;
 class ClassDecl;
 class ClangModuleLoader;
 class DependentMemberType;
@@ -899,20 +900,24 @@ class alignas(1 << TypeAlignInBits) TypeBase
   bool satisfiesClassConstraint();
 
   /// Determine whether this type can be used as a base type for AST
-  /// name lookup, which is the case for nominal types, protocol compositions
-  /// and archetypes.
+  /// name lookup, which is the case for nominal types, existential types,
+  /// archetypes, and tuples.
   ///
   /// Generally, the static vs instance and mutating vs nonmutating distinction
   /// is handled elsewhere, so metatypes, lvalue types and inout types are not
   /// allowed here.
   ///
-  /// Similarly, tuples formally have members, but this does not go through
-  /// name lookup.
+  /// Tuples have formal members to project elements by index or by label; these
+  /// are handled directly by Sema and do not go through name lookup.
+  ///
+  /// Bona fide members on tuples are defined on extensions of
+  /// Builtin.TheTupleType.
   bool mayHaveMembers() {
     return (is<ArchetypeType>() ||
             is<ModuleType>() ||
             isExistentialType() ||
-            getAnyNominal());
+            getAnyNominal() ||
+            is<TupleType>());
   }
 
   /// Checks whether this type may potentially be callable. This returns true
@@ -5489,6 +5494,23 @@ BEGIN_CAN_TYPE_WRAPPER(ExistentialType, Type)
   PROXY_CAN_TYPE_SIMPLE_GETTER(getConstraintType)
 END_CAN_TYPE_WRAPPER(ExistentialType, Type)
 
+
+/// BuiltinTupleType - A singleton nominal type which serves as the declared
+/// interface type of Builtin.TheTupleType.
+class BuiltinTupleType : public NominalType {
+public:
+  // Implement isa/cast/dyncast/etc.
+  static bool classof(const TypeBase *T) {
+    return T->getKind() == TypeKind::BuiltinTuple;
+  }
+
+private:
+  friend class ASTContext;
+  BuiltinTupleType(BuiltinTupleDecl *TheDecl, const ASTContext &Ctx);
+};
+BEGIN_CAN_TYPE_WRAPPER(BuiltinTupleType, NominalType)
+END_CAN_TYPE_WRAPPER(BuiltinTupleType, NominalType)
+
 /// 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.

lib/AST/ASTContext.cpp

@@ -552,6 +552,12 @@ struct ASTContext::Implementation {
 
   /// Memory allocation arena for the term rewriting system.
   std::unique_ptr<rewriting::RewriteContext> TheRewriteContext;
+
+  /// The singleton Builtin.TheTupleType.
+  BuiltinTupleDecl *TheTupleTypeDecl = nullptr;
+
+  /// The declared interface type of Builtin.TheTupleType.
+  BuiltinTupleType *TheTupleType = nullptr;
 };
 
 ASTContext::Implementation::Implementation()
@@ -3173,6 +3179,14 @@ PackType *PackType::get(const ASTContext &C, ArrayRef<Type> elements) {
   RecursiveTypeProperties properties;
   bool isCanonical = true;
   for (Type eltTy : elements) {
+    assert(!eltTy->isTypeParameter() ||
+           !eltTy->getRootGenericParam()->isTypeSequence() &&
+           "Pack type parameter outside of a pack expansion");
+    assert(!eltTy->is<SequenceArchetypeType>() &&
+           "Pack type archetype outside of a pack expansion");
+    assert(!eltTy->is<PackType>() &&
+           "Cannot have pack directly inside another pack");
+
     properties |= eltTy->getRecursiveProperties();
     if (!eltTy->isCanonical())
       isCanonical = false;
@@ -4511,6 +4525,11 @@ Type ExistentialType::get(Type constraint) {
                                                 properties);
 }
 
+BuiltinTupleType::BuiltinTupleType(BuiltinTupleDecl *TheDecl,
+                                   const ASTContext &Ctx)
+  : NominalType(TypeKind::BuiltinTuple, &Ctx, TheDecl, Type(),
+                RecursiveTypeProperties()) { }
+
 LValueType *LValueType::get(Type objectTy) {
   assert(!objectTy->is<LValueType>() && !objectTy->is<InOutType>() &&
          "cannot have 'inout' or @lvalue wrapped inside an @lvalue");
@@ -5914,3 +5933,30 @@ bool ASTContext::isASCIIString(StringRef s) const {
   }
   return true;
 }
+
+/// The special Builtin.TheTupleType, which parents tuple extensions and
+/// conformances.
+BuiltinTupleDecl *ASTContext::getBuiltinTupleDecl() {
+  auto &result = getImpl().TheTupleTypeDecl;
+
+  if (result)
+    return result;
+
+  result = new (*this) BuiltinTupleDecl(Id_TheTupleType,
+                                        TheBuiltinModule->getFiles()[0]);
+  result->setAccess(AccessLevel::Public);
+
+  return result;
+}
+
+/// The declared interface type of Builtin.TheTupleType.
+BuiltinTupleType *ASTContext::getBuiltinTupleType() {
+  auto &result = getImpl().TheTupleType;
+
+  if (result)
+    return result;
+
+  result = new (*this) BuiltinTupleType(getBuiltinTupleDecl(), *this);
+
+  return result;
+}

lib/AST/ASTDumper.cpp

@@ -847,6 +847,11 @@ namespace {
       printCommonPost(CD);
     }
 
+    void visitBuiltinTupleDecl(BuiltinTupleDecl *BTD) {
+      printCommon(BTD, "builtin_tuple_decl");
+      printCommonPost(BTD);
+    }
+
     void visitPatternBindingDecl(PatternBindingDecl *PBD) {
       printCommon(PBD, "pattern_binding_decl");
 
@@ -3800,6 +3805,12 @@ namespace {
       PrintWithColorRAII(OS, ParenthesisColor) << ')';
     }
 
+    void visitBuiltinTupleType(BuiltinTupleType *T, StringRef label) {
+      printCommon(label, "builtin_tuple_type");
+      printField("decl", T->getDecl()->printRef());
+      PrintWithColorRAII(OS, ParenthesisColor) << ')';
+    }
+
     void visitMetatypeType(MetatypeType *T, StringRef label) {
       printCommon(label, "metatype_type");
       if (T->hasRepresentation())

lib/AST/ASTMangler.cpp

@@ -1330,7 +1330,8 @@ void ASTMangler::appendType(Type type, GenericSignature sig,
     case TypeKind::Struct:
     case TypeKind::BoundGenericClass:
     case TypeKind::BoundGenericEnum:
-    case TypeKind::BoundGenericStruct: {
+    case TypeKind::BoundGenericStruct:
+    case TypeKind::BuiltinTuple: {
       GenericTypeDecl *Decl;
       if (auto typeAlias = dyn_cast<TypeAliasType>(type.getPointer()))
         Decl = typeAlias->getDecl();
@@ -2473,6 +2474,9 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl) {
     return;
   }
 
+  if (nominal && isa<BuiltinTupleDecl>(nominal))
+    return appendOperator("BT");
+
   appendContextOf(decl);
 
   // Always use Clang names for imported Clang declarations, unless they don't
@@ -2554,6 +2558,8 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl) {
     case DeclKind::Struct:
       appendOperator("V");
       break;
+    case DeclKind::BuiltinTuple:
+      llvm_unreachable("Not implemented");
     }
   }
 

lib/AST/ASTPrinter.cpp

@@ -3673,6 +3673,11 @@ void PrintAST::visitProtocolDecl(ProtocolDecl *decl) {
                        Options.BracketOptions.shouldCloseNominal(decl));
   }
 }
+
+void PrintAST::visitBuiltinTupleDecl(BuiltinTupleDecl *decl) {
+  llvm_unreachable("Not implemented");
+}
+
 static bool isStructOrClassContext(DeclContext *dc) {
   auto *nominal = dc->getSelfNominalTypeDecl();
   if (nominal == nullptr)
@@ -5563,13 +5568,6 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
     Printer.callPrintStructurePre(PrintStructureKind::TupleType);
     SWIFT_DEFER { Printer.printStructurePost(PrintStructureKind::TupleType); };
 
-    // Single-element tuples can only appear in SIL mode.
-    if (T->getNumElements() == 1 &&
-        !T->getElement(0).hasName() &&
-        !T->getElementType(0)->is<PackExpansionType>()) {
-      Printer << "@tuple ";
-    }
-
     Printer << "(";
 
     auto Fields = T->getElements();
@@ -5587,6 +5585,10 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
       if (TD.hasName()) {
         Printer.printName(TD.getName(), PrintNameContext::TupleElement);
         Printer << ": ";
+      } else if (e == 1 && !EltType->is<PackExpansionType>()) {
+        // Unlabeled one-element tuples always print the empty label to
+        // distinguish them from the older syntax for ParenType.
+        Printer << "_: ";
       }
       visit(EltType);
     }
@@ -6334,6 +6336,10 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
     }
   }
 
+  void visitBuiltinTupleType(BuiltinTupleType *T) {
+    printQualifiedType(T);
+  }
+
   void visitLValueType(LValueType *T) {
     Printer << "@lvalue ";
     visit(T->getObjectType());

lib/AST/ASTScopeCreation.cpp

@@ -331,6 +331,11 @@ class NodeAdder
   VISIT_AND_CREATE_WHOLE_PORTION(OpaqueTypeDecl, OpaqueTypeScope)
 #undef VISIT_AND_CREATE_WHOLE_PORTION
 
+  ASTScopeImpl *visitBuiltinTupleDecl(BuiltinTupleDecl *btd, ASTScopeImpl *p,
+                                      ScopeCreator &scopeCreator) {
+    llvm_unreachable("BuiltinTupleDecl should never appear in a source file");
+  }
+
   // This declaration is handled from
   // addChildrenForParsedAccessors
   ASTScopeImpl *visitAccessorDecl(AccessorDecl *ad, ASTScopeImpl *p,