Default value for optional ctor params should be used when matching prop is ignored

[layomia]

Repro:

public class Point
{
    public int X { get; }

    [JsonIgnore]
    public int Y { get; }

    public Point(int x, int y = 5) => (X, Y) = (x, y);
}

string json = "{}";
var point = JsonSerializer.Deserialize<Point>(json);
Console.WriteLine(point.Y);
// Expected: 5
// Actual: 0

The behavior should be consistent with then the matching prop is not ignored:

public class Point
{
    public int X { get; }

    public int Y { get; }

    public Point(int x, int y = 5) => (X, Y) = (x, y);
}

string json = "{}";
var point = JsonSerializer.Deserialize<Point>(json);
Console.WriteLine(point.Y);
// Expected: 5
// Actual: 5

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Issue Details

---

Repro:

public class Point
{
    public int X { get; }

    [JsonIgnore]
    public int Y { get; }

    public Point(int x, int y = 5) => (X, Y) = (x, y);
}

string json = "{}";
var point = JsonSerializer.Deserialize<Point>(json);
Console.WriteLine(point.Y);
// Expected: 5
// Actual: 0

The behavior should be consistent with then the matching prop is not ignored:

public class Point
{
    public int X { get; }

    public int Y { get; }

    public Point(int x, int y = 5) => (X, Y) = (x, y);
}

string json = "{}";
var point = JsonSerializer.Deserialize<Point>(json);
Console.WriteLine(point.Y);
// Expected: 5
// Actual: 5

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json
Milestone:	7.0.0

[krwq]

should we consider this for 6.0? it might lead to data loss

[eiriktsarpalis]

should we consider this for 6.0? it might lead to data loss

Is this a regression from 5.0 behavior?

[layomia]

Is this a regression from 5.0 behavior?

No this behavior shipped in 5.0. A fix for this can be considered a breaking change. I recommend addressing early in 7.0 as a bug fix so we have time to react to any feedback.