func test(_ collection: RandomAccessCollection) { ``` ___ -This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value iff the following criteria hold:+This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value if and only if the following criteria hold: * The type of the invoked member (accessor — for storage declarations), as viewed in context of the *base type*, must **not** contain references to `Self` or `Self`-rooted associated types in [non-covariant](https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)) position.

 func test(_ collection: RandomAccessCollection) { ``` ___ -This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value iff the following criteria hold:+This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value if the following criteria hold:

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(syntax_kind, is_token=False, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    elif 'List' in syntax_kind:+      buildable_type = syntax_kind + 'Syntax'+    else:+      buildable_type = 'TokenSyntax'++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]

 bool ASTContext::isASCIIString(StringRef s) const {   }   return true; }++/// Classify usages of Self in the given type.+///+/// \param position The position we are currently at, in terms of variance.+static SelfReferenceInfo+findExistentialSelfReferences(GenericSignature existentialSig, Type type,+                              SelfReferencePosition position) {+  // If there are no type parameters, we're done.+  if (!type->hasTypeParameter())+    return SelfReferenceInfo();++  // Tuples preserve variance.+  if (auto tuple = type->getAs<TupleType>()) {+    auto info = SelfReferenceInfo();+    for (auto &elt : tuple->getElements()) {+      info |= findExistentialSelfReferences(existentialSig, elt.getType(),+                                            position);+    }++    // A covariant Self result inside a tuple will not be bona fide.+    info.hasCovariantSelfResult = false;++    return info;+  }++  // Function preserve variance in the result type, and flip variance in+  // the parameter type.+  if (auto funcTy = type->getAs<AnyFunctionType>()) {+    auto inputInfo = SelfReferenceInfo();+    for (auto param : funcTy->getParams()) {+      // inout parameters are invariant.+      if (param.isInOut()) {+        inputInfo |=+            findExistentialSelfReferences(existentialSig, param.getPlainType(),+                                          SelfReferencePosition::Invariant);+        continue;+      }+      inputInfo |= findExistentialSelfReferences(+          existentialSig, param.getParameterType(), position.flipped());+    }++    // A covariant Self result inside a parameter will not be bona fide.+    inputInfo.hasCovariantSelfResult = false;++    auto resultInfo = findExistentialSelfReferences(+        existentialSig, funcTy->getResult(), position);+    if (resultInfo.selfRef == SelfReferencePosition::Covariant) {+      resultInfo.hasCovariantSelfResult = true;+    }+    return inputInfo |= resultInfo;+  }++  // Metatypes preserve variance.+  if (auto metaTy = type->getAs<MetatypeType>()) {+    return findExistentialSelfReferences(existentialSig,+                                         metaTy->getInstanceType(), position);+  }++  // Optionals preserve variance.+  if (auto optType = type->getOptionalObjectType()) {+    return findExistentialSelfReferences(existentialSig, optType, position);+  }++  // DynamicSelfType preserves variance.+  // FIXME: This shouldn't ever appear in protocol requirement+  // signatures.+  if (auto selfType = type->getAs<DynamicSelfType>()) {+    return findExistentialSelfReferences(existentialSig,+                                         selfType->getSelfType(), position);+  }++  // Bound generic types are invariant.+  if (auto boundGenericType = type->getAs<BoundGenericType>()) {

 func test(_ collection: RandomAccessCollection) { ``` ___ -This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value iff the following criteria hold:+This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value if the following criteria hold:

 func test(_ collection: RandomAccessCollection) { ``` ___ -This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value iff the following criteria hold:+This way, a protocol or protocol extension member (method/property/subscript/initializer) may be used on an existential value if the following criteria hold:

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(type_name, syntax_kind, is_token, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    else:+      buildable_type = type_name++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]+}++%     if kind == 'Syntax':+public protocol ${kind}Buildable: ${kind}ListBuildable {+%     else:+public protocol ${kind}Buildable: SyntaxBuildable, ${kind}ListBuildable {+%     end+  func build${kind}(format: Format, leadingTrivia: Trivia?) -> ${build_kind}+}++extension ${kind}Buildable {+%     if kind != 'Syntax':+  func build${kind}(format: Format) -> ${build_kind} {+    build${kind}(format: format, leadingTrivia: nil)+  }+%     end+  public func buildSyntax(format: Format) -> Syntax {+    buildSyntax(format: format, leadingTrivia: nil)+  }++  public func buildSyntax(format: Format, leadingTrivia: Trivia?) -> Syntax {+    Syntax(build${kind}(format: format, leadingTrivia: leadingTrivia))+  }++  public func build${kind}List(format: Format) -> [${build_kind}] {+    build${kind}List(format: format, leadingTrivia: nil)+  }++  public func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}] {+    [build${kind}(format: format, leadingTrivia: leadingTrivia)]+  }+}++%   end+% end++// MARK: - Buildables++% for node in SYNTAX_NODES:+%   if node.is_buildable():+public struct ${node.syntax_kind}: ${node.base_kind}Buildable {

+import XCTest+import SwiftSyntax+import SwiftSyntaxBuilder++final class EnumCaseElementTests: XCTestCase {+  func test_enumCaseElementInit() {+    let leadingTrivia = Trivia.garbageText("␣")+    let string = SyntaxFactory.makeStringSegment("Hello World")+    let segment = StringSegment(content: string)+    let segments = StringLiteralSegments([segment])++    let stringLiteralExpr: ExprBuildable = StringLiteralExpr(openDelimiter: nil,+                                                             openQuote: SyntaxFactory.makeStringQuoteToken(),

+import XCTest+import SwiftSyntax+import SwiftSyntaxBuilder++final class EnumCaseElementTests: XCTestCase {+  func test_enumCaseElementInit() {+    let leadingTrivia = Trivia.garbageText("␣")+    let string = SyntaxFactory.makeStringSegment("Hello World")+    let segment = StringSegment(content: string)+    let segments = StringLiteralSegments([segment])++    let stringLiteralExpr: ExprBuildable = StringLiteralExpr(openDelimiter: nil,+                                                             openQuote: SyntaxFactory.makeStringQuoteToken(),

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(type_name, syntax_kind, is_token, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    else:+      buildable_type = type_name++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]+}++%     if kind == 'Syntax':+public protocol ${kind}Buildable: ${kind}ListBuildable {+%     else:+public protocol ${kind}Buildable: SyntaxBuildable, ${kind}ListBuildable {+%     end+  func build${kind}(format: Format, leadingTrivia: Trivia?) -> ${build_kind}+}++extension ${kind}Buildable {+%     if kind != 'Syntax':+  func build${kind}(format: Format) -> ${build_kind} {+    build${kind}(format: format, leadingTrivia: nil)+  }+%     end+  public func buildSyntax(format: Format) -> Syntax {+    buildSyntax(format: format, leadingTrivia: nil)+  }++  public func buildSyntax(format: Format, leadingTrivia: Trivia?) -> Syntax {+    Syntax(build${kind}(format: format, leadingTrivia: leadingTrivia))+  }++  public func build${kind}List(format: Format) -> [${build_kind}] {+    build${kind}List(format: format, leadingTrivia: nil)+  }++  public func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}] {+    [build${kind}(format: format, leadingTrivia: leadingTrivia)]+  }+}++%   end+% end++// MARK: - Buildables++% for node in SYNTAX_NODES:+%   if node.is_buildable():+public struct ${node.syntax_kind}: ${node.base_kind}Buildable {

+import XCTest+import SwiftSyntax+import SwiftSyntaxBuilder++final class EnumCaseElementTests: XCTestCase {+  func test_enumCaseElementInit() {+    let leadingTrivia = Trivia.garbageText("␣")+    let string = SyntaxFactory.makeStringSegment("Hello World")+    let segment = StringSegment(content: string)+    let segments = StringLiteralSegments([segment])++    let stringLiteralExpr: ExprBuildable = StringLiteralExpr(openDelimiter: nil,+                                                             openQuote: SyntaxFactory.makeStringQuoteToken(),

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(syntax_kind, is_token=False, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    elif 'List' in syntax_kind:+      buildable_type = syntax_kind + 'Syntax'+    else:+      buildable_type = 'TokenSyntax'++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]+}++%     if kind == 'Syntax':+public protocol ${kind}Buildable: ${kind}ListBuildable {+%     else:+public protocol ${kind}Buildable: SyntaxBuildable, ${kind}ListBuildable {+%     end+  func build${kind}(format: Format, leadingTrivia: Trivia?) -> ${build_kind}+}++extension ${kind}Buildable {+  public func buildSyntax(format: Format, leadingTrivia: Trivia?) -> Syntax {+    Syntax(build${kind}(format: format, leadingTrivia: leadingTrivia))+  }++  public func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}] {+    [build${kind}(format: format, leadingTrivia: leadingTrivia)]+  }+}++%   end+% end++// MARK: - Buildables++% for node in SYNTAX_NODES:+%   if node.is_buildable():+%     return_type = 'Syntax'+%     if node.base_kind != 'Syntax':+%       return_type = node.base_kind + 'Syntax'+%     end+public struct ${node.syntax_kind}: ${node.base_kind}Buildable {+%     child_params = []+%     for child in node.children:+%       param_type = syntax_buildable_child_type(child.syntax_kind, child.is_token(), child.is_optional)+%       child_params.append("%s: %s" % (child.swift_name, param_type))+%     end+%     for child_param in child_params:+  let ${child_param}+%     end++  public init(+    ${',\n    '.join(child_params)}+  ) {+%     for child in node.children:+    self.${child.swift_name} = ${child.swift_name}+%     end+  }+  +  func build${node.syntax_kind}(format: Format, leadingTrivia: Trivia?) -> ${node.syntax_kind}Syntax {+%     child_params = []+%     for child in node.children:+%       param_name = child.swift_name+%       if child.is_token() and child.requires_leading_newline:+%         param_name = param_name + '.withLeadingTrivia(.newlines(1) + format._makeIndent())'+%       elif child.syntax_kind in SYNTAX_BASE_KINDS or not child.is_token():+%         if child.is_optional:+%           param_name = param_name + "?"+%         end+%         format = 'format'+%         if child.is_indented:+%           format += '._indented()'+%         end            +%         param_name = param_name + ".build" + child.syntax_kind + "(format: " + format + ", leadingTrivia: nil)"+%       end+%       child_params.append("%s: %s" % (child.swift_name, param_name))+%     end+    let ${node.swift_syntax_kind} = SyntaxFactory.make${node.syntax_kind}(+      ${',\n      '.join(child_params)}+    )+    +    if let leadingTrivia = leadingTrivia {+      return ${node.swift_syntax_kind}+        .withLeadingTrivia(leadingTrivia)+    }++    return ${node.swift_syntax_kind}+  }++  public func build${node.base_kind}(format: Format, leadingTrivia: Trivia?) -> ${return_type} {

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(syntax_kind, is_token=False, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    elif 'List' in syntax_kind:+      buildable_type = syntax_kind + 'Syntax'+    else:+      buildable_type = 'TokenSyntax'++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]+}++%     if kind == 'Syntax':+public protocol ${kind}Buildable: ${kind}ListBuildable {+%     else:+public protocol ${kind}Buildable: SyntaxBuildable, ${kind}ListBuildable {+%     end+  func build${kind}(format: Format, leadingTrivia: Trivia?) -> ${build_kind}+}++extension ${kind}Buildable {+  public func buildSyntax(format: Format, leadingTrivia: Trivia?) -> Syntax {+    Syntax(build${kind}(format: format, leadingTrivia: leadingTrivia))+  }++  public func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}] {+    [build${kind}(format: format, leadingTrivia: leadingTrivia)]+  }+}++%   end+% end++// MARK: - Buildables++% for node in SYNTAX_NODES:+%   if node.is_buildable():+%     return_type = 'Syntax'+%     if node.base_kind != 'Syntax':+%       return_type = node.base_kind + 'Syntax'+%     end

+%{+  from gyb_syntax_support import *+  from gyb_syntax_support.kinds import lowercase_first_word+  from gyb_syntax_support.kinds import SYNTAX_BASE_KINDS+  # -*- mode: Swift -*-+  # Ignore the following admonition it applies to the resulting .swift file only++  def syntax_buildable_child_type(syntax_kind, is_token=False, is_optional=False):+    if syntax_kind in SYNTAX_BASE_KINDS:+      buildable_type = syntax_kind + 'Buildable'+    elif not is_token:+      buildable_type = syntax_kind+    elif 'List' in syntax_kind:+      buildable_type = syntax_kind + 'Syntax'+    else:+      buildable_type = 'TokenSyntax'++    if is_optional:+      buildable_type += '?'++    return buildable_type+}%+//// Automatically Generated From DeclBuildables.swift.gyb.+//===----------------------------------------------------------------------===//+//+// This source file is part of the Swift.org open source project+//+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors+// Licensed under Apache License v2.0 with Runtime Library Exception+//+// See https://swift.org/LICENSE.txt for license information+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors+//+//===----------------------------------------------------------------------===//++import SwiftSyntax++// MARK: Protocols++% for kind in SYNTAX_BASE_KINDS:+%   if kind != 'SyntaxCollection':+%     build_kind = 'Syntax' if kind == 'Syntax' else kind + 'Syntax'+%     if kind == 'Syntax':+public protocol ${kind}ListBuildable {+%     else:+public protocol ${kind}ListBuildable: SyntaxListBuildable {+%     end+  func build${kind}List(format: Format, leadingTrivia: Trivia?) -> [${build_kind}]