Add Missing Definitions of canDerive{En,De}codable

lib/Sema/DerivedConformanceCodable.cpp

@@ -1071,10 +1071,35 @@ static bool canSynthesize(DerivedConformance &derived, ValueDecl *requirement) {
     LLVM_FALLTHROUGH;
   case CodingKeysClassification::Valid:
     return true;
+static bool canDeriveCodable(NominalTypeDecl *NTD,
+                             KnownProtocolKind Kind) {
+  assert(Kind == KnownProtocolKind::Encodable ||
+         Kind == KnownProtocolKind::Decodable);
+
+  // Structs and classes can explicitly derive Encodable and Decodable
+  // conformance (explicitly meaning we can synthesize an implementation if
+  // a type conforms manually).
+  // FIXME: Enums too!
+  if (!isa<StructDecl>(NTD) && !isa<ClassDecl>(NTD)) {
+    return false;
   }
+
+  auto *PD = NTD->getASTContext().getProtocol(Kind);
+  if (!PD) {
+    return false;
+  }
+
   return true;
 }
 
+bool DerivedConformance::canDeriveDecodable(NominalTypeDecl *NTD) {
+  return canDeriveCodable(NTD, KnownProtocolKind::Decodable);
+}
+
+bool DerivedConformance::canDeriveEncodable(NominalTypeDecl *NTD) {
+  return canDeriveCodable(NTD, KnownProtocolKind::Encodable);
+}
+
 ValueDecl *DerivedConformance::deriveEncodable(ValueDecl *requirement) {
   // We can only synthesize Encodable for structs and classes.
   if (!isa<StructDecl>(Nominal) && !isa<ClassDecl>(Nominal))

lib/Sema/DerivedConformances.cpp

@@ -88,6 +88,14 @@ bool DerivedConformance::derivesProtocolConformance(DeclContext *DC,
   if (*derivableKind == KnownDerivableProtocolKind::Differentiable)
     return true;
 
+  if (*derivableKind == KnownDerivableProtocolKind::Encodable) {
+    return canDeriveEncodable(Nominal);
+  }
+
+  if (*derivableKind == KnownDerivableProtocolKind::Decodable) {
+    return canDeriveDecodable(Nominal);
+  }
+
   if (auto *enumDecl = dyn_cast<EnumDecl>(Nominal)) {
     switch (*derivableKind) {
         // The presence of a raw type is an explicit declaration that
@@ -137,33 +145,15 @@ bool DerivedConformance::derivesProtocolConformance(DeclContext *DC,
       default:
         return false;
     }
-  } else if (isa<StructDecl>(Nominal) || isa<ClassDecl>(Nominal)) {
-    // Structs and classes can explicitly derive Encodable and Decodable
-    // conformance (explicitly meaning we can synthesize an implementation if
-    // a type conforms manually).
-    if (*derivableKind == KnownDerivableProtocolKind::Encodable ||
-        *derivableKind == KnownDerivableProtocolKind::Decodable) {
-      // FIXME: This is not actually correct. We cannot promise to always
-      // provide a witness here for all structs and classes. Unfortunately,
-      // figuring out whether this is actually possible requires much more
-      // context -- a TypeChecker and the parent decl context at least -- and is
-      // tightly coupled to the logic within DerivedConformance.
-      // This unfortunately means that we expect a witness even if one will not
-      // be produced, which requires DerivedConformance::deriveCodable to output
-      // its own diagnostics.
-      return true;
-    }
-
-    // Structs can explicitly derive Equatable conformance.
-    if (isa<StructDecl>(Nominal)) {
+  } else if (isa<StructDecl>(Nominal)) {
     switch (*derivableKind) {
     case KnownDerivableProtocolKind::Equatable:
+      // Structs can explicitly derive Equatable conformance.
       return canDeriveEquatable(DC, Nominal);
     default:
       return false;
     }
   }
-  }
   return false;
 }
 

lib/Sema/DerivedConformances.h

@@ -278,6 +278,11 @@ public:
   /// \returns the derived member, which will also be added to the type.
   ValueDecl *deriveBridgedNSError(ValueDecl *requirement);
 
+  /// Determine if \c Encodable can be derived for the given type.
+  static bool canDeriveEncodable(NominalTypeDecl *NTD);
+  /// Determine if \c Decodable can be derived for the given type.
+  static bool canDeriveDecodable(NominalTypeDecl *NTD);
+
   /// Derive a CodingKey requirement for an enum type.
   ///
   /// \returns the derived member, which will also be added to the type.