[Sema] Properly handle missing coding keys in enum Codable synthesis

rdar://79671408

Author: drexin

lib/Sema/DerivedConformanceCodable.cpp

@@ -459,6 +459,15 @@ static bool validateCaseCodingKeysEnum(const DerivedConformance &derived,
                           ? addImplicitCaseCodingKeys(
                               enumDecl, elementDecl, codingKeysEnum)
                           : caseCodingKeysDecls.front();
+
+  if (!result) {
+    // There is no coding key defined for this element,
+    // which is okay, because not all elements have to
+    // be considered for serialization. Attempts to
+    // en-/decode them will be handled at runtime.
+    return true;
+  }
+
   auto *codingKeysTypeDecl = dyn_cast<TypeDecl>(result);
   if (!codingKeysTypeDecl) {
     result->diagnose(diag::codable_codingkeys_type_is_not_an_enum_here,
@@ -625,7 +634,7 @@ static ThrowStmt *createThrowCodingErrorStmt(ASTContext &C, Expr *containerExpr,
   assert(contextDecl && "Missing Context decl.");
 
   auto *debugMessageExpr = new (C) StringLiteralExpr(
-      StringRef("Invalid number of keys found, expected one."), SourceRange(),
+      C.AllocateCopy(debugMessage), SourceRange(),
       /* Implicit */ true);
 
   auto *contextTypeExpr =
@@ -1069,14 +1078,14 @@ deriveBodyEncodable_enum_encode(AbstractFunctionDecl *encodeDecl, void *) {
           // This case should not be encodable, so throw an error if an attempt
           // is made to encode it
           auto debugMessage =
-              "Case '" + elt->getBaseIdentifier().str() +
-              "' cannot be decoded because it is not defined in CodingKeys.";
+              "Case '" + elt->getBaseIdentifier().str().str() +
+              "' cannot be encoded because it is not defined in CodingKeys.";
           auto *selfRefExpr = new (C) DeclRefExpr(
               ConcreteDeclRef(selfRef), DeclNameLoc(), /* Implicit */ true);
 
           auto *throwStmt = createThrowCodingErrorStmt(
               C, containerExpr, C.getEncodingErrorDecl(), C.Id_invalidValue,
-              selfRefExpr, debugMessage.str());
+              selfRefExpr, debugMessage);
 
           caseStatements.push_back(throwStmt);
         } else {

test/decl/protocol/special/coding/enum_codable_exclude_element.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift
+
+enum EnumWithExcludedElement : Codable {
+    case x
+    case y
+
+    enum CodingKeys: CodingKey {
+        case x
+    }
+}