`JsonObjectCreationHandling.Populate` handles init-only properties inconsistently in reflection/source gen

[eiriktsarpalis]

Consider the following reproduction:

using System.Text.Json;
using System.Text.Json.Serialization;

string json = """{ "Nested" : { "Id" : "id" } }""";

MyPoco? result = JsonSerializer.Deserialize<MyPoco>(json);
Console.WriteLine(result!.Nested.Id); // "id"
result = JsonSerializer.Deserialize<MyPoco>(json, MyContext.Default.MyPoco);
Console.WriteLine(result!.Nested.Id); // System.InvalidOperationException: Setting init-only properties is not supported in source generation mode.

[JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
public class MyPoco
{
    public NestedPoco Nested { get; } = new();
}

public class NestedPoco
{
    public string Id { get; init; } = "<null>";
}

[JsonSerializable(typeof(MyPoco))]
public partial class MyContext : JsonSerializerContext { }

This is most probably expected given the nature of init-only properties in source gen, but perhaps it would make sense to also fail in the case of the reflection serializer (since the current behavior violates the init contract of the nested poco).

[ghost]

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Issue Details

---

Consider the following reproduction: ```C# using System.Text.Json; using System.Text.Json.Serialization; string json = """{ "Nested" : { "Id" : "id" } }"""; MyPoco? result = JsonSerializer.Deserialize(json); Console.WriteLine(result!.Nested.Id); // "id" result = JsonSerializer.Deserialize(json, MyContext.Default.MyPoco); Console.WriteLine(result!.Nested.Id); // System.InvalidOperationException: Setting init-only properties is not supported in source generation mode. [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)] public class MyPoco { public NestedPoco Nested { get; } = new(); } public class NestedPoco { public string Id { get; init; } = ""; } [JsonSerializable(typeof(MyPoco))] public partial class MyContext : JsonSerializerContext { } ```

Author:	eiriktsarpalis
Assignees:	-
Labels:	`area-System.Text.Json`, `untriaged`
Milestone:	-

[krwq]

something to consider when fixing this: should reflection work when Nested is initially null?