Sema: Allow non-final classes to satisfy properties and subscripts with covariant Self

include/swift/AST/Decl.h

@@ -4123,12 +4123,14 @@ struct SelfReferenceKind {
     return SelfReferenceKind(false, false, false, false);
   }
 
-  /// The type refers to 'Self', but only as the result type of a method.
+  /// The type refers to 'Self', but only as the type of a property or
+  /// the result type of a method/subscript.
   static SelfReferenceKind Result() {
     return SelfReferenceKind(true, false, false, false);
   }
 
-  /// The type refers to 'Self', but only as the parameter type of a method.
+  /// The type refers to 'Self', but only as the parameter type
+  /// of a method/subscript.
   static SelfReferenceKind Parameter() {
     return SelfReferenceKind(false, true, false, false);
   }

include/swift/AST/DiagnosticsSema.def

@@ -1992,14 +1992,15 @@ NOTE(witness_self_weaken_same_type,none,
       "consider weakening the same-type requirement %0 == %1 to a superclass "
       "requirement", (Type, Type))
 ERROR(witness_requires_dynamic_self,none,
-      "method %0 in non-final class %1 must return 'Self' to conform to "
-      "protocol %2",
-      (DeclName, Type, Type))
+      "%select{%error|method|property|subscript}0 %1 in non-final class %2 "
+      "must %select{%error|return|specify type|return}0 'Self' "
+      "to conform to protocol %3",
+      (RequirementKind, DeclName, Type, Type))
 ERROR(witness_requires_class_implementation,none,
-      "method %0 in non-final class %1 cannot be implemented in a "
-      "protocol extension because it returns 'Self' and has associated type "
-      "requirements",
-      (DeclName, Type))
+      "%select{%error|method|%error|subscript}0 %1 in non-final class %2 "
+      "cannot be implemented in a protocol extension because it returns 'Self' "
+      "and has associated type requirements",
+      (RequirementKind, DeclName, Type))
 ERROR(witness_not_accessible_proto,none,
       "%select{initializer %1|method %1|%select{|setter for }2property %1"
       "|subscript%select{| setter}2}0 must be declared "

lib/AST/Decl.cpp

@@ -4965,12 +4965,13 @@ ProtocolDecl::findProtocolSelfReferences(const ValueDecl *value,
     return ::findProtocolSelfReferences(this, type,
                                         skipAssocTypes);
   } else {
-    if (::findProtocolSelfReferences(this, type,
-                                     skipAssocTypes)) {
-      return SelfReferenceKind::Other();
-    }
-    return SelfReferenceKind::None();
+    assert(isa<VarDecl>(value));
+
+    return ::findProtocolSelfReferences(this, type,
+                                        skipAssocTypes);
   }
+
+  return SelfReferenceKind::None();
 }
 
 bool ProtocolDecl::isAvailableInExistential(const ValueDecl *decl) const {

lib/Sema/CSApply.cpp

@@ -1061,7 +1061,6 @@ namespace {
                          AccessSemantics semantics) {
       auto choice = overload.choice;
       auto openedType = overload.openedType;
-      auto openedFullType = overload.openedFullType;
 
       ValueDecl *member = choice.getDecl();
 
@@ -1097,13 +1096,15 @@ namespace {
         return result;
       }
 
+      auto refTy = simplifyType(overload.openedFullType);
+
       // If we're referring to the member of a module, it's just a simple
       // reference.
       if (baseTy->is<ModuleType>()) {
         assert(semantics == AccessSemantics::Ordinary &&
                "Direct property access doesn't make sense for this");
         auto ref = new (context) DeclRefExpr(memberRef, memberLoc, Implicit);
-        cs.setType(ref, simplifyType(openedFullType));
+        cs.setType(ref, refTy);
         ref->setFunctionRefKind(choice.getFunctionRefKind());
         auto *DSBI = cs.cacheType(new (context) DotSyntaxBaseIgnoredExpr(
             base, dotLoc, ref, cs.getType(ref)));
@@ -1114,8 +1115,6 @@ namespace {
         (!baseIsInstance && member->isInstanceMember());
       bool isPartialApplication = shouldBuildCurryThunk(choice, baseIsInstance);
 
-      auto refTy = simplifyType(openedFullType);
-
       // The formal type of the 'self' value for the member's declaration.
       Type containerTy = getBaseType(refTy->castTo<FunctionType>());
 
@@ -1278,8 +1277,8 @@ namespace {
           = new (context) MemberRefExpr(base, dotLoc, memberRef,
                                         memberLoc, Implicit, semantics);
         memberRefExpr->setIsSuper(isSuper);
+        cs.setType(memberRefExpr, refTy->castTo<FunctionType>()->getResult());
 
-        cs.setType(memberRefExpr, simplifyType(openedType));
         Expr *result = memberRefExpr;
         closeExistential(result, locator);
 

lib/Sema/TypeCheckProtocol.cpp

@@ -2078,7 +2078,7 @@ static Type getRequirementTypeForDisplay(ModuleDecl *module,
     return FunctionType::get(params, result, fnTy->getExtInfo());
   }
 
-  return substType(type, /*result*/false);
+  return substType(type, /*result*/ true);
 }
 
 diag::RequirementKind
@@ -3343,41 +3343,36 @@ void ConformanceChecker::checkNonFinalClassWitness(ValueDecl *requirement,
         emitDeclaredHereIfNeeded(diags, diagLoc, witness);
       });
   } else if (selfKind.result) {
-    // The reference to Self occurs in the result type. A non-final class
-    // can satisfy this requirement with a method that returns Self.
+    // The reference to Self occurs in the result type of a method/subscript
+    // or the type of a property. A non-final class can satisfy this requirement
+    // by holding onto Self accordingly.
+    if (witness->getDeclContext()->getSelfClassDecl()) {
+      const bool hasDynamicSelfResult = [&] {
+        if (auto func = dyn_cast<AbstractFunctionDecl>(witness)) {
+          return func->hasDynamicSelfResult();
+        } else if (auto var = dyn_cast<VarDecl>(witness)) {
+          return var->getInterfaceType()->hasDynamicSelfType();
+        }
 
-    // If the function has a dynamic Self, it's okay.
-    if (auto func = dyn_cast<FuncDecl>(witness)) {
-      if (func->getDeclContext()->getSelfClassDecl() &&
-          !func->hasDynamicSelfResult()) {
+        return cast<SubscriptDecl>(witness)
+            ->getElementInterfaceType()
+            ->hasDynamicSelfType();
+      }();
+
+      if (!hasDynamicSelfResult) {
         diagnoseOrDefer(requirement, false,
           [witness, requirement](NormalProtocolConformance *conformance) {
             auto proto = conformance->getProtocol();
             auto &diags = proto->getASTContext().Diags;
             SourceLoc diagLoc = getLocForDiagnosingWitness(conformance,witness);
-            diags.diagnose(diagLoc,
-                           diag::witness_requires_dynamic_self,
+            diags.diagnose(diagLoc, diag::witness_requires_dynamic_self,
+                           getProtocolRequirementKind(requirement),
                            requirement->getName(),
                            conformance->getType(),
                            proto->getDeclaredInterfaceType());
             emitDeclaredHereIfNeeded(diags, diagLoc, witness);
           });
       }
-
-    // Constructors conceptually also have a dynamic Self
-    // return type, so they're okay.
-    } else if (!isa<ConstructorDecl>(witness)) {
-      diagnoseOrDefer(requirement, false,
-        [witness, requirement](NormalProtocolConformance *conformance) {
-          auto proto = conformance->getProtocol();
-          auto &diags = proto->getASTContext().Diags;
-          SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);
-          diags.diagnose(diagLoc, diag::witness_self_non_subtype,
-                         proto->getDeclaredInterfaceType(),
-                         requirement->getName(),
-                         conformance->getType());
-          emitDeclaredHereIfNeeded(diags, diagLoc, witness);
-        });
     }
   } else if (selfKind.requirement) {
     if (auto targetPair = getAdopteeSelfSameTypeConstraint(classDecl,
@@ -3413,8 +3408,8 @@ void ConformanceChecker::checkNonFinalClassWitness(ValueDecl *requirement,
   // constraint that either the requirement not produce 'Self' in a
   // covariant position, or the type of the requirement does not involve
   // associated types.
-  if (auto func = dyn_cast<FuncDecl>(witness)) {
-    if (func->getDeclContext()->getExtendedProtocolDecl()) {
+  if (isa<FuncDecl>(witness) || isa<SubscriptDecl>(witness)) {
+    if (witness->getDeclContext()->getExtendedProtocolDecl()) {
       auto selfKindWithAssocTypes = Proto->findProtocolSelfReferences(
           requirement,
           /*allowCovariantParameters=*/false,
@@ -3427,6 +3422,7 @@ void ConformanceChecker::checkNonFinalClassWitness(ValueDecl *requirement,
             auto &diags = proto->getASTContext().Diags;
             diags.diagnose(conformance->getLoc(),
                            diag::witness_requires_class_implementation,
+                           getProtocolRequirementKind(requirement),
                            requirement->getName(),
                            conformance->getType());
             diags.diagnose(witness, diag::decl_declared_here,