Merge pull request #38216 from drexin/wip-fix-codable-enum-5.5

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

Author: DougGregor

lib/Sema/DerivedConformanceCodable.cpp

@@ -488,6 +488,15 @@ 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,

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
+     }
+ }