[Bug]: Default ObjectToInferredTypesConverter causes TypeDiscriminator property to be missing in top-level types for polymorphic JSON Serialization

[Greenscreener]

Describe the bug 🐞

When using the SystemTextJsonContentSerializer with default JsonSerializerOptions including the ObjectToInferredTypesConverter, TypeDiscriminator properties used in Polymorphic JSON Serialization are ignored in first-level objects, even though nested types receive their TypeDiscriminator properties correctly.

Step to reproduce

1. Use the default SystemTextJsonContentSerializer with default configuration.
2. Create a type with a polymorphic serilaization configiuration, for instance:

   [JsonPolymorphic(TypeDiscriminatorPropertyName = "type")]
   [JsonDerivedType(typeof(UserIdentifier), typeDiscriminator: "m.id.user")]
   [JsonDerivedType(typeof(PhoneIdentifier), typeDiscriminator: "m.id.phone")]
   [JsonDerivedType(typeof(ThirdpartyIdentifier), typeDiscriminator: "m.id.thirdparty")]
   public abstract record Identifier();
   public record UserIdentifier(string user) : Identifier();
   public record PhoneIdentifier(string country, string phone) : Identifier();
   public record ThirdpartyIdentifier(string medium, string address) : Identifier();

3. Try to send such a type using a Post method, it gets serialized correctly but the TypeDiscriminator property is ignored.

Reproduction repository

No response

Expected behavior

TypeDiscriminator property should be present

Screenshots 🖼️

No response

IDE

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Operating system

No response

Version

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Device

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Refit Version

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Additional information ℹ️

No response

[etiennelepagel]

Hey @Greenscreener,

You can use this:

var jsonSerializerOptions = new JsonSerializerOptions(JsonSerializerDefaults.Web)
{
    PropertyNameCaseInsensitive = true
};

this._targetsProvider = RestService.For<IMyClient>(
    this.HttpClient,
    new RefitSettings { ContentSerializer = new SystemTextJsonContentSerializer(jsonSerializerOptions) });

In my case PropertyNameCaseInsensitive = true was essential to deserialize the response, but the type descriminator is still there without it.