Revert "Sema: Don't need to derive CaseIterable's AllCases associated type"

This reverts commit 3ae31d5.

Author: CodaFi

lib/Sema/DerivedConformanceCaseIterable.cpp

@@ -69,6 +69,15 @@ static ArraySliceType *computeAllCasesType(NominalTypeDecl *enumDecl) {
   return ArraySliceType::get(enumType);
 }
 
+static Type deriveCaseIterable_AllCases(DerivedConformance &derived) {
+  // enum SomeEnum : CaseIterable {
+  //   @derived
+  //   typealias AllCases = [SomeEnum]
+  // }
+  auto *rawInterfaceType = computeAllCasesType(cast<EnumDecl>(derived.Nominal));
+  return derived.getConformanceContext()->mapTypeIntoContext(rawInterfaceType);
+}
+
 ValueDecl *DerivedConformance::deriveCaseIterable(ValueDecl *requirement) {
   // Conformance can't be synthesized in an extension.
   if (checkAndDiagnoseDisallowedContext(requirement))
@@ -102,3 +111,18 @@ ValueDecl *DerivedConformance::deriveCaseIterable(ValueDecl *requirement) {
 
   return propDecl;
 }
+
+Type DerivedConformance::deriveCaseIterable(AssociatedTypeDecl *assocType) {
+  // Check that we can actually derive CaseIterable for this type.
+  if (!canDeriveConformance(Nominal))
+    return nullptr;
+
+  if (assocType->getName() == Context.Id_AllCases) {
+    return deriveCaseIterable_AllCases(*this);
+  }
+
+  Context.Diags.diagnose(assocType->getLoc(),
+                         diag::broken_case_iterable_requirement);
+  return nullptr;
+}
+

lib/Sema/DerivedConformances.h

@@ -163,6 +163,12 @@ class DerivedConformance {
   /// \returns the derived member, which will also be added to the type.
   ValueDecl *deriveCaseIterable(ValueDecl *requirement);
 
+  /// Derive a CaseIterable type witness for an enum if it has no associated
+  /// values for any of its cases.
+  ///
+  /// \returns the derived member, which will also be added to the type.
+  Type deriveCaseIterable(AssociatedTypeDecl *assocType);
+
   /// Determine if a RawRepresentable requirement can be derived for a type.
   ///
   /// This is implemented for non-empty enums without associated values,

lib/Sema/TypeCheckProtocol.cpp

@@ -5763,6 +5763,8 @@ TypeChecker::deriveTypeWitness(DeclContext *DC,
   switch (*knownKind) {
   case KnownProtocolKind::RawRepresentable:
     return std::make_pair(derived.deriveRawRepresentable(AssocType), nullptr);
+  case KnownProtocolKind::CaseIterable:
+    return std::make_pair(derived.deriveCaseIterable(AssocType), nullptr);
   case KnownProtocolKind::Differentiable:
     return derived.deriveDifferentiable(AssocType);
   default:

stdlib/public/core/CompilerProtocols.swift

@@ -249,7 +249,7 @@ extension RawRepresentable where RawValue: Hashable, Self: Hashable {
 /// demonstrates this automatic implementation.
 public protocol CaseIterable {
   /// A type that can represent a collection of all values of this type.
-  associatedtype AllCases: Collection = [Self]
+  associatedtype AllCases: Collection
     where AllCases.Element == Self
 
   /// A collection of all values of this type.

test/IDE/complete_override.swift

@@ -896,10 +896,11 @@ struct SynthesizedConformance3: Hashable {
 enum SynthesizedConformance4: CaseIterable {
   case a, b, c, d
   #^OVERRIDE_SYNTHESIZED_4^#
-// OVERRIDE_SYNTHESIZED_4: Begin completions, 3 items
+// OVERRIDE_SYNTHESIZED_4: Begin completions, 4 items
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[InstanceVar]/Super/IsSystem:       var hashValue: Int
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[InstanceMethod]/Super/IsSystem:    func hash(into hasher: inout Hasher) {|};
 // OVERRIDE_SYNTHESIZED_4-DAG: Decl[StaticVar]/Super/IsSystem:         static var allCases: [SynthesizedConformance4];
+// OVERRIDE_SYNTHESIZED_4-DAG: Decl[AssociatedType]/Super/IsSystem:    typealias AllCases = {#(Type)#};
 }
 
 class SynthesizedConformance5: SynthesizedConformance2 {