[Sema] Use decodeIfPresent for optional enum params in Codable synthesis

rdar://79584261

Author: drexin

lib/Sema/DerivedConformanceCodable.cpp

@@ -1674,8 +1674,16 @@ deriveBodyDecodable_enum_init(AbstractFunctionDecl *initDecl, void *) {
           }
 
           // Type.self
-          auto *parameterTypeExpr = TypeExpr::createImplicit(
-              funcDC->mapTypeIntoContext(paramDecl->getInterfaceType()), C);
+          auto varType = funcDC->mapTypeIntoContext(
+                  paramDecl->getValueInterfaceType());
+
+          bool useIfPresentVariant = false;
+          if (auto objType = varType->getOptionalObjectType()) {
+            varType = objType;
+            useIfPresentVariant = true;
+          }
+
+          auto *parameterTypeExpr = TypeExpr::createImplicit(varType, C);
           auto *parameterMetaTypeExpr =
               new (C) DotSelfExpr(parameterTypeExpr, SourceLoc(), SourceLoc());
           // BarCodingKeys.x
@@ -1688,9 +1696,12 @@ deriveBodyDecodable_enum_init(AbstractFunctionDecl *initDecl, void *) {
           auto *nestedContainerExpr = new (C)
               DeclRefExpr(ConcreteDeclRef(nestedContainerDecl), DeclNameLoc(),
                           /*Implicit=*/true, AccessSemantics::DirectToStorage);
+
           // decode(_:, forKey:)
+          auto methodName = useIfPresentVariant ? C.Id_decodeIfPresent
+                                                : C.Id_decode;
           auto *decodeCall = UnresolvedDotExpr::createImplicit(
-              C, nestedContainerExpr, C.Id_decode, {Identifier(), C.Id_forKey});
+              C, nestedContainerExpr, methodName, {Identifier(), C.Id_forKey});
 
           // nestedContainer.decode(Type.self, forKey: BarCodingKeys.x)
           auto *callExpr = CallExpr::createImplicit(

test/stdlib/CodableTests.swift

@@ -597,6 +597,39 @@ class TestCodable : TestCodableSuper {
         }
     }
 
+    // MARK: - Optional
+    func test_Optional_enum_JSON() {
+        enum A: Codable, Equatable {
+            case a(Int?)
+        }
+
+        enum B: Codable, Equatable {
+            case b(Int? = nil)
+        }
+
+        enum C: Codable, Equatable {
+            case c(x: Int?)
+        }
+
+        enum D: Codable, Equatable {
+            case d(x: Int? = nil)
+        }
+
+        expectRoundTripEqualityThroughJSON(for: A.a(1), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: A.a(nil), lineNumber: #line)
+
+        expectRoundTripEqualityThroughJSON(for: B.b(2), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: B.b(nil), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: B.b(), lineNumber: #line)
+
+        expectRoundTripEqualityThroughJSON(for: C.c(x: 3), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: C.c(x: nil), lineNumber: #line)
+
+        expectRoundTripEqualityThroughJSON(for: D.d(x: 4), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: D.d(x: nil), lineNumber: #line)
+        expectRoundTripEqualityThroughJSON(for: D.d(), lineNumber: #line)
+    }
+
     // MARK: - PartialRangeFrom
     func test_PartialRangeFrom_JSON() {
         let value = 0...
@@ -899,6 +932,7 @@ var tests = [
     "test_Locale_Plist" : TestCodable.test_Locale_Plist,
     "test_NSRange_JSON" : TestCodable.test_NSRange_JSON,
     "test_NSRange_Plist" : TestCodable.test_NSRange_Plist,
+    "test_Optional_enum_JSON" : TestCodable.test_Optional_enum_JSON,
     "test_PartialRangeFrom_JSON" : TestCodable.test_PartialRangeFrom_JSON,
     "test_PartialRangeFrom_Plist" : TestCodable.test_PartialRangeFrom_Plist,
     "test_PartialRangeThrough_JSON" : TestCodable.test_PartialRangeThrough_JSON,