JsonConstrutor different behavior between Newtonsoft.Json and System.Text.Json

[NN---]

Description

    public class Q
    {
        [JsonConstructor]
        public Q(int? x)
        {
            if (x is null) throw new Exception();
            X = x.Value;
        }
        
        public int X { get; }
    }

Given string:

var str = JsonSerializer.Deserialize<Q>("{\"X\":123}" ");

It is deserialized fine with Newtonsoft, but not with System.Text.
The reason is that constructor parameter type is int? while the property has type of int.
Newtonsoft doesn't validate it, giving an option to do anything in the constructor.

There are possible workarounds but they are not as simple as the original code:

Make property private and add a public non nullable:

    public class Q
    {
        [JsonConstructor]
        public Q(int? x)
        {
            if (x is null) throw new Exception();
            X = x.Value;
        }

        public int? X { get; }

        [JsonIgnore]
        public int NonNullableX => X!.Value;
    }

Annotate nullable as non nullable, however it requires explicit call to Value.:

    public class Q
    {
        [JsonConstructor]
        public Q(int? x)
        {
            if (x is null) throw new Exception();
            X = x.Value;
        }

        [DisallowNull][NotNull]
        public int? X { get; }
    }

Configuration

Regression?

Not a regression.

Other information

[Clockwork-Muse]

Um, if you throw an exception like that, why not just make the constructor take a non-nullable?

[NN---]

It is a general question.
The argument validation can be more complicated than this.

[layomia]

Dup of #44428.