Fix deserialization of nullable structs with constructors. (#86896)

Author: eiriktsarpalis

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/NullableConverter.cs

@@ -10,6 +10,7 @@ internal sealed class NullableConverter<T> : JsonConverter<T?> where T : struct
         internal override Type? ElementType => typeof(T);
         public override bool HandleNull => true;
         internal override bool CanPopulate => _elementConverter.CanPopulate;
+        internal override bool ConstructorIsParameterized => _elementConverter.ConstructorIsParameterized;
 
         // It is possible to cache the underlying converter since this is an internal converter and
         // an instance is created only once for each JsonSerializerOptions instance.
@@ -20,6 +21,7 @@ public NullableConverter(JsonConverter<T> elementConverter)
             _elementConverter = elementConverter;
             IsInternalConverterForNumberType = elementConverter.IsInternalConverterForNumberType;
             ConverterStrategy = elementConverter.ConverterStrategy;
+            ConstructorInfo = elementConverter.ConstructorInfo;
         }
 
         internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options, scoped ref ReadStack state, out T? value)

src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.AttributePresence.cs

@@ -160,5 +160,16 @@ public async Task Struct_Use_DefaultCtor_ByDefault()
             Assert.Equal(1, point2.X);
             Assert.Equal(2, point2.Y);
         }
+
+        [Fact]
+        public async Task CanDeserializeNullableStructWithCtor()
+        {
+            string json = @"{""X"":1,""Y"":2}";
+
+            Point_2D_Struct_WithAttribute? point2 = await Serializer.DeserializeWrapper<Point_2D_Struct_WithAttribute?>(json);
+            Assert.NotNull(point2);
+            Assert.Equal(1, point2.Value.X);
+            Assert.Equal(2, point2.Value.Y);
+        }
     }
 }

src/libraries/System.Text.Json/tests/Common/PropertyVisibilityTests.InitOnly.cs

@@ -64,6 +64,20 @@ public virtual async Task NonPublicInitOnlySetter_With_JsonInclude(Type type)
             Assert.Equal(@"{""MyInt"":1}", await Serializer.SerializeWrapper(obj));
         }
 
+        [Fact]
+        public async Task NullableStructWithInitOnlyProperty()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/86483
+
+            StructWithInitOnlyProperty? value = new StructWithInitOnlyProperty { MyInt = 42 };
+            string json = await Serializer.SerializeWrapper(value);
+
+            Assert.Equal("""{"MyInt":42}""", json);
+
+            StructWithInitOnlyProperty? deserializedValue = await Serializer.DeserializeWrapper<StructWithInitOnlyProperty?>(json);
+            Assert.Equal(deserializedValue, value);
+        }
+
         public class ClassWithInitOnlyProperty
         {
             public int MyInt { get; init; }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/ConstructorTests.cs

@@ -58,6 +58,7 @@ protected ConstructorTests_Metadata(JsonSerializerWrapper stringWrapper)
         [JsonSerializable(typeof(Parameterized_WrapperForICollection))]
         [JsonSerializable(typeof(Point_2D_Struct))]
         [JsonSerializable(typeof(Point_2D_Struct_WithAttribute))]
+        [JsonSerializable(typeof(Point_2D_Struct_WithAttribute?))]
         [JsonSerializable(typeof(ObjWCtorMixedParams))]
         [JsonSerializable(typeof(Person_Class))]
         [JsonSerializable(typeof(Point_2D))]
@@ -200,6 +201,7 @@ public ConstructorTests_Default(JsonSerializerWrapper jsonSerializer) : base(jso
         [JsonSerializable(typeof(Parameterized_WrapperForICollection))]
         [JsonSerializable(typeof(Point_2D_Struct))]
         [JsonSerializable(typeof(Point_2D_Struct_WithAttribute))]
+        [JsonSerializable(typeof(Point_2D_Struct_WithAttribute?))]
         [JsonSerializable(typeof(ObjWCtorMixedParams))]
         [JsonSerializable(typeof(Person_Class))]
         [JsonSerializable(typeof(Point_2D))]

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/PropertyVisibilityTests.cs

@@ -162,6 +162,7 @@ public override async Task HonorJsonPropertyName_PrivateSetter()
         [JsonSerializable(typeof(ClassWithMissingObjectProperty))]
         [JsonSerializable(typeof(ClassWithInitOnlyProperty))]
         [JsonSerializable(typeof(StructWithInitOnlyProperty))]
+        [JsonSerializable(typeof(StructWithInitOnlyProperty?))]
         [JsonSerializable(typeof(ClassWithCustomNamedInitOnlyProperty))]
         [JsonSerializable(typeof(StructWithCustomNamedInitOnlyProperty))]
         [JsonSerializable(typeof(MyClassWithValueTypeInterfaceProperty))]
@@ -406,6 +407,7 @@ public void PublicContextAndJsonSerializerOptions()
         [JsonSerializable(typeof(ClassWithMissingObjectProperty))]
         [JsonSerializable(typeof(ClassWithInitOnlyProperty))]
         [JsonSerializable(typeof(StructWithInitOnlyProperty))]
+        [JsonSerializable(typeof(StructWithInitOnlyProperty?))]
         [JsonSerializable(typeof(ClassWithCustomNamedInitOnlyProperty))]
         [JsonSerializable(typeof(StructWithCustomNamedInitOnlyProperty))]
         [JsonSerializable(typeof(MyClassWithValueTypeInterfaceProperty))]