Derive Equatable & Hashable for uninhabited types

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -92,10 +92,6 @@ static bool canDeriveConformance(TypeChecker &tc, DeclContext *DC,
                                  ProtocolDecl *protocol) {
   // The type must be an enum or a struct.
   if (auto enumDecl = dyn_cast<EnumDecl>(target)) {
-    // The enum must have cases.
-    if (!enumDecl->hasCases())
-      return false;
-
     // The cases must not have associated values, or all associated values must
     // conform to the protocol.
     return allAssociatedValuesConformToProtocol(tc, DC, enumDecl, protocol);
@@ -304,6 +300,34 @@ static GuardStmt *returnIfNotEqualGuard(ASTContext &C,
   return new (C) GuardStmt(SourceLoc(), C.AllocateCopy(conditions), body);
 }
 
+static void
+deriveBodyEquatable_enum_uninhabited_eq(AbstractFunctionDecl *eqDecl) {
+  auto parentDC = eqDecl->getDeclContext();
+  ASTContext &C = parentDC->getASTContext();
+
+  auto args = eqDecl->getParameterLists().back();
+  auto aParam = args->get(0);
+  auto bParam = args->get(1);
+
+  assert(!cast<EnumDecl>(aParam->getType()->getAnyNominal())->hasCases());
+
+  SmallVector<ASTNode, 1> statements;
+  SmallVector<ASTNode, 0> cases;
+
+  // switch (a, b) { }
+  auto aRef = new (C) DeclRefExpr(aParam, DeclNameLoc(), /*implicit*/ true);
+  auto bRef = new (C) DeclRefExpr(bParam, DeclNameLoc(), /*implicit*/ true);
+  auto abExpr = TupleExpr::create(C, SourceLoc(), {aRef, bRef}, {}, {},
+                                  SourceLoc(), /*HasTrailingClosure*/ false,
+                                  /*implicit*/ true);
+  auto switchStmt = SwitchStmt::create(LabeledStmtInfo(), SourceLoc(), abExpr,
+                                       SourceLoc(), cases, SourceLoc(), C);
+  statements.push_back(switchStmt);
+
+  auto body = BraceStmt::create(C, SourceLoc(), statements, SourceLoc());
+  eqDecl->setBody(body);
+}
+
 /// Derive the body for an '==' operator for an enum that has no associated
 /// values. This generates code that converts each value to its integer ordinal
 /// and compares them, which produces an optimal single icmp instruction.
@@ -673,11 +697,13 @@ ValueDecl *DerivedConformance::deriveEquatable(ValueDecl *requirement) {
 
   // Build the necessary decl.
   if (requirement->getBaseName() == "==") {
-    if (auto ED = dyn_cast<EnumDecl>(Nominal)) {
+    if (auto ed = dyn_cast<EnumDecl>(Nominal)) {
       auto bodySynthesizer =
-          ED->hasOnlyCasesWithoutAssociatedValues()
-              ? &deriveBodyEquatable_enum_noAssociatedValues_eq
-              : &deriveBodyEquatable_enum_hasAssociatedValues_eq;
+          !ed->hasCases()
+              ? &deriveBodyEquatable_enum_uninhabited_eq
+              : ed->hasOnlyCasesWithoutAssociatedValues()
+                    ? &deriveBodyEquatable_enum_noAssociatedValues_eq
+                    : &deriveBodyEquatable_enum_hasAssociatedValues_eq;
       return deriveEquatable_eq(*this, TC.Context.Id_derived_enum_equals,
                                 bodySynthesizer);
     } else if (isa<StructDecl>(Nominal))

test/Sema/enum_conformance_synthesis.swift

@@ -45,7 +45,7 @@ enum CustomHashable {
 
   var hashValue: Int { return 0 }
 }
-func ==(x: CustomHashable, y: CustomHashable) -> Bool { // expected-note 5 {{non-matching type}}
+func ==(x: CustomHashable, y: CustomHashable) -> Bool { // expected-note 4 {{non-matching type}}
   return true
 }
 
@@ -63,7 +63,7 @@ enum InvalidCustomHashable {
 
   var hashValue: String { return "" } // expected-note{{previously declared here}}
 }
-func ==(x: InvalidCustomHashable, y: InvalidCustomHashable) -> String { // expected-note 5 {{non-matching type}}
+func ==(x: InvalidCustomHashable, y: InvalidCustomHashable) -> String { // expected-note 4 {{non-matching type}}
   return ""
 }
 func invalidCustomHashable() {
@@ -173,8 +173,8 @@ func genericNotHashable() {
   GenericNotHashable<String>.A("a").hash(into: &hasher) // expected-error {{value of type 'GenericNotHashable<String>' has no member 'hash'}}
 }
 
-// An enum with no cases should not derive conformance.
-enum NoCases: Hashable {} // expected-error 2 {{does not conform}}
+// An enum with no cases should also derive conformance.
+enum NoCases: Hashable {}
 
 // rdar://19773050
 private enum Bar<T> {
@@ -213,7 +213,7 @@ public enum Medicine {
 
 extension Medicine : Equatable {}
 
-public func ==(lhs: Medicine, rhs: Medicine) -> Bool { // expected-note 5 {{non-matching type}}
+public func ==(lhs: Medicine, rhs: Medicine) -> Bool { // expected-note 4 {{non-matching type}}
   return true
 }
 
@@ -236,7 +236,7 @@ extension NotExplicitlyHashableAndCannotDerive : CaseIterable {} // expected-err
 // Verify that conformance (albeit manually implemented) can still be added to
 // a type in a different file.
 extension OtherFileNonconforming: Hashable {
-  static func ==(lhs: OtherFileNonconforming, rhs: OtherFileNonconforming) -> Bool { // expected-note 5 {{non-matching type}}
+  static func ==(lhs: OtherFileNonconforming, rhs: OtherFileNonconforming) -> Bool { // expected-note 4 {{non-matching type}}
     return true
   }
   var hashValue: Int { return 0 }