[Sema] Implement type checking for the _typeEraser attribute

include/swift/AST/Attr.h

@@ -1117,6 +1117,9 @@ class TypeEraserAttr final : public DeclAttribute {
         TypeEraserLoc(typeEraserLoc) {}
 
   const TypeLoc &getTypeEraserLoc() const { return TypeEraserLoc; }
+  TypeLoc &getTypeEraserLoc() { return TypeEraserLoc; }
+
+  bool hasViableTypeEraserInit(ProtocolDecl *protocol) const;
 
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DAK_TypeEraser;

include/swift/AST/DiagnosticsSema.def

@@ -4412,6 +4412,39 @@ ERROR(dynamic_replacement_replaced_constructor_is_convenience, none,
 ERROR(dynamic_replacement_replaced_constructor_is_not_convenience, none,
       "replaced constructor %0 is not marked as convenience", (DeclNameRef))
 
+//------------------------------------------------------------------------------
+// MARK: @_typeEraser()
+//------------------------------------------------------------------------------
+
+ERROR(non_nominal_type_eraser,none,
+      "type eraser must be a class, struct, or enum", ())
+ERROR(type_eraser_does_not_conform,none,
+      "type eraser %0 must conform to protocol %1", (Type, Type))
+ERROR(type_eraser_not_accessible,none,
+      "%select{private|fileprivate|internal|public|open}0 type eraser %1 "
+      "cannot have more restrictive access than protocol %2 "
+      "(which is %select{private|fileprivate|internal|public|open}3)",
+      (AccessLevel, Identifier, Type, AccessLevel))
+ERROR(type_eraser_missing_init,none,
+      "type eraser %0 must have an initializer of the form "
+      "'init<T: %1>(erasing: T)'", (Type, StringRef))
+ERROR(type_eraser_unviable_init,none,
+      "type eraser %0 has no viable initializer of the form "
+      "'init<T: %1>(erasing: T)'", (Type, StringRef))
+
+NOTE(type_eraser_declared_here,none,
+     "type eraser declared here",())
+NOTE(type_eraser_failable_init,none,
+     "'init(erasing:)' cannot be failable",())
+NOTE(type_eraser_init_unsatisfied_requirements,none,
+     "'init(erasing:)' cannot have unsatisfied requirements "
+     "when %0 = 'some %1'", (Type, StringRef))
+NOTE(type_eraser_init_not_accessible,none,
+     "%select{private|fileprivate|internal|public|open}0 'init(erasing:)' "
+     "cannot have more restrictive access than protocol %1 "
+     "(which is %select{private|fileprivate|internal|public|open}2)",
+     (AccessLevel, Type, AccessLevel))
+
 //------------------------------------------------------------------------------
 // MARK: @available
 //------------------------------------------------------------------------------

include/swift/AST/KnownIdentifiers.def

@@ -64,6 +64,7 @@ IDENTIFIER(encode)
 IDENTIFIER(encodeIfPresent)
 IDENTIFIER(Encoder)
 IDENTIFIER(encoder)
+IDENTIFIER(erasing)
 IDENTIFIER(error)
 IDENTIFIER(errorDomain)
 IDENTIFIER(first)

include/swift/AST/TypeCheckRequests.h

@@ -2054,6 +2054,25 @@ class DifferentiableAttributeTypeCheckRequest
   void cacheResult(IndexSubset *value) const;
 };
 
+/// Checks whether a type eraser has a viable initializer.
+class TypeEraserHasViableInitRequest
+    : public SimpleRequest<TypeEraserHasViableInitRequest,
+                           bool(TypeEraserAttr *, ProtocolDecl *),
+                           CacheKind::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  // Evaluation
+  llvm::Expected<bool> evaluate(Evaluator &evaluator, TypeEraserAttr *attr,
+                                ProtocolDecl *protocol) const;
+
+public:
+  bool isCached() const { return true; }
+};
+
 // Allow AnyValue to compare two Type values, even though Type doesn't
 // support ==.
 template<>

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -46,6 +46,9 @@ SWIFT_REQUEST(TypeChecker, DefaultTypeRequest,
 SWIFT_REQUEST(TypeChecker, DifferentiableAttributeTypeCheckRequest,
               IndexSubset *(DifferentiableAttr *),
               SeparatelyCached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, TypeEraserHasViableInitRequest,
+              bool(TypeEraserAttr *, ProtocolDecl *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, DynamicallyReplacedDeclRequest,
               ValueDecl *(ValueDecl *),
               Cached, NoLocationInfo)

lib/AST/Attr.cpp

@@ -1333,6 +1333,14 @@ SourceLoc DynamicReplacementAttr::getRParenLoc() const {
   return getTrailingLocations()[1];
 }
 
+bool
+TypeEraserAttr::hasViableTypeEraserInit(ProtocolDecl *protocol) const {
+  return evaluateOrDefault(protocol->getASTContext().evaluator,
+                           TypeEraserHasViableInitRequest{
+                               const_cast<TypeEraserAttr *>(this), protocol},
+                           false);
+}
+
 AvailableAttr *
 AvailableAttr::createPlatformAgnostic(ASTContext &C,
                                    StringRef Message,

lib/Sema/TypeCheckAttr.cpp

@@ -108,7 +108,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(StaticInitializeObjCMetadata)
   IGNORED_ATTR(SynthesizedProtocol)
   IGNORED_ATTR(Testable)
-  IGNORED_ATTR(TypeEraser)
   IGNORED_ATTR(WeakLinked)
   IGNORED_ATTR(PrivateImport)
   IGNORED_ATTR(DisfavoredOverload)
@@ -239,6 +238,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
   void visitDiscardableResultAttr(DiscardableResultAttr *attr);
   void visitDynamicReplacementAttr(DynamicReplacementAttr *attr);
+  void visitTypeEraserAttr(TypeEraserAttr *attr);
   void visitImplementsAttr(ImplementsAttr *attr);
 
   void visitFrozenAttr(FrozenAttr *attr);
@@ -2376,6 +2376,179 @@ void AttributeChecker::visitDynamicReplacementAttr(DynamicReplacementAttr *attr)
   }
 }
 
+llvm::Expected<bool>
+TypeEraserHasViableInitRequest::evaluate(Evaluator &evaluator,
+                                         TypeEraserAttr *attr,
+                                         ProtocolDecl *protocol) const {
+  auto &ctx = protocol->getASTContext();
+  auto &diags = ctx.Diags;
+  TypeLoc &typeEraserLoc = attr->getTypeEraserLoc();
+  TypeRepr *typeEraserRepr = typeEraserLoc.getTypeRepr();
+  DeclContext *dc = protocol->getDeclContext();
+  Type protocolType = protocol->getDeclaredType();
+
+  // Get the NominalTypeDecl for the type eraser.
+  Type typeEraser = typeEraserLoc.getType();
+  if (!typeEraser && typeEraserRepr) {
+    auto resolution = TypeResolution::forContextual(protocol);
+    typeEraser = resolution.resolveType(typeEraserRepr, /*options=*/None);
+    typeEraserLoc.setType(typeEraser);
+  }
+
+  if (typeEraser->hasError())
+    return false;
+
+  // The type eraser must be a concrete nominal type
+  auto nominalTypeDecl = typeEraser->getAnyNominal();
+  if (auto typeAliasDecl = dyn_cast_or_null<TypeAliasDecl>(nominalTypeDecl))
+    nominalTypeDecl = typeAliasDecl->getUnderlyingType()->getAnyNominal();
+
+  if (!nominalTypeDecl || isa<ProtocolDecl>(nominalTypeDecl)) {
+    diags.diagnose(typeEraserLoc.getLoc(), diag::non_nominal_type_eraser);
+    return false;
+  }
+
+  // The nominal type must be accessible wherever the protocol is accessible
+  if (nominalTypeDecl->getFormalAccess() < protocol->getFormalAccess()) {
+    diags.diagnose(typeEraserLoc.getLoc(), diag::type_eraser_not_accessible,
+                   nominalTypeDecl->getFormalAccess(), nominalTypeDecl->getName(),
+                   protocolType, protocol->getFormalAccess());
+    diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
+    return false;
+  }
+
+  // The type eraser must conform to the annotated protocol
+  if (!TypeChecker::conformsToProtocol(typeEraser, protocol, dc, None)) {
+    diags.diagnose(typeEraserLoc.getLoc(), diag::type_eraser_does_not_conform,
+                   typeEraser, protocolType);
+    diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
+    return false;
+  }
+
+  // The type eraser must have an init of the form init<T: Protocol>(erasing: T)
+  auto lookupResult = TypeChecker::lookupMember(dc, typeEraser,
+                                                DeclNameRef::createConstructor());
+
+  // Keep track of unviable init candidates for diagnostics
+  enum class UnviableReason {
+    Failable,
+    UnsatisfiedRequirements,
+    Inaccessible,
+  };
+  SmallVector<std::tuple<ConstructorDecl *, UnviableReason, Type>, 2> unviable;
+
+  bool foundMatch = llvm::any_of(lookupResult, [&](const LookupResultEntry &entry) {
+    auto *init = cast<ConstructorDecl>(entry.getValueDecl());
+    if (!init->isGeneric() || init->getGenericParams()->size() != 1)
+      return false;
+
+    auto genericSignature = init->getGenericSignature();
+    auto genericParamType = genericSignature->getInnermostGenericParams().front();
+
+    // Fow now, only allow one parameter.
+    auto params = init->getParameters();
+    if (params->size() != 1)
+      return false;
+
+    // The parameter must have the form `erasing: T` where T conforms to the protocol.
+    ParamDecl *param = *init->getParameters()->begin();
+    if (param->getArgumentName() != ctx.Id_erasing ||
+        !param->getInterfaceType()->isEqual(genericParamType) ||
+        !genericSignature->conformsToProtocol(genericParamType, protocol))
+      return false;
+
+    // Allow other constraints as long as the init can be called with any
+    // type conforming to the annotated protocol. We will check this by
+    // substituting the protocol's Self type for the generic arg and check that
+    // the requirements in the generic signature are satisfied.
+    auto baseMap =
+        typeEraser->getContextSubstitutionMap(nominalTypeDecl->getParentModule(),
+                                              nominalTypeDecl);
+    QuerySubstitutionMap getSubstitution{baseMap};
+    auto subMap = SubstitutionMap::get(
+        genericSignature,
+        [&](SubstitutableType *type) -> Type {
+          if (type->isEqual(genericParamType))
+            return protocol->getSelfTypeInContext();
+
+          return getSubstitution(type);
+        },
+        TypeChecker::LookUpConformance(dc));
+
+    // Use invalid 'SourceLoc's to suppress diagnostics.
+    auto result = TypeChecker::checkGenericArguments(
+          protocol, SourceLoc(), SourceLoc(), typeEraser,
+          genericSignature->getGenericParams(),
+          genericSignature->getRequirements(),
+          QuerySubstitutionMap{subMap},
+          TypeChecker::LookUpConformance(dc),
+          None);
+
+    if (result != RequirementCheckResult::Success) {
+      unviable.push_back(
+          std::make_tuple(init, UnviableReason::UnsatisfiedRequirements,
+                          genericParamType));
+      return false;
+    }
+
+    if (init->isFailable()) {
+      unviable.push_back(
+          std::make_tuple(init, UnviableReason::Failable, genericParamType));
+      return false;
+    }
+
+    if (init->getFormalAccess() < protocol->getFormalAccess()) {
+      unviable.push_back(
+          std::make_tuple(init, UnviableReason::Inaccessible, genericParamType));
+      return false;
+    }
+
+    return true;
+  });
+
+  if (!foundMatch) {
+    if (unviable.empty()) {
+      diags.diagnose(attr->getLocation(), diag::type_eraser_missing_init,
+                     typeEraser, protocol->getName().str());
+      diags.diagnose(nominalTypeDecl->getLoc(), diag::type_eraser_declared_here);
+      return false;
+    }
+
+    diags.diagnose(attr->getLocation(), diag::type_eraser_unviable_init,
+                   typeEraser, protocol->getName().str());
+    for (auto &candidate: unviable) {
+      auto init = std::get<0>(candidate);
+      auto reason = std::get<1>(candidate);
+      auto genericParamType = std::get<2>(candidate);
+
+      switch (reason) {
+      case UnviableReason::Failable:
+        diags.diagnose(init->getLoc(), diag::type_eraser_failable_init);
+        break;
+      case UnviableReason::UnsatisfiedRequirements:
+        diags.diagnose(init->getLoc(),
+                       diag::type_eraser_init_unsatisfied_requirements,
+                       genericParamType, protocol->getName().str());
+        break;
+      case UnviableReason::Inaccessible:
+        diags.diagnose(init->getLoc(), diag::type_eraser_init_not_accessible,
+                       init->getFormalAccess(), protocolType,
+                       protocol->getFormalAccess());
+        break;
+      }
+    }
+    return false;
+  }
+
+  return true;
+}
+
+void AttributeChecker::visitTypeEraserAttr(TypeEraserAttr *attr) {
+  assert(isa<ProtocolDecl>(D));
+  // Invoke the request.
+  (void)attr->hasViableTypeEraserInit(cast<ProtocolDecl>(D));
+}
+
 void AttributeChecker::visitImplementsAttr(ImplementsAttr *attr) {
   TypeLoc &ProtoTypeLoc = attr->getProtocolType();