JSON serializer source generator produces warnings for types with nullable generic parameters

[Tragetaschen]

Description

When the model type has a constructor with a generic type that has a nullable type argument, that nullability is not reproduced in the source-generated code for the JSON serializer and produces a nullability warning

Reproduction Steps

In a net6.0 classlib project:

using System.Text.Json.Serialization;

namespace JsonSerializerNullability;

public class Model
{
    public Model(Dictionary<string, object?> property) => Property = property;
    public Dictionary<string, object?> Property { get; }
}

[JsonSerializable(typeof(Model))]
internal partial class MyJsonSerializerContext : JsonSerializerContext
{ }

Expected behavior

No warnings should be produced

Actual behavior

Warning	CS8620	Argument of type 'Dictionary<string, object>' cannot be used for parameter 'property' of type 'Dictionary<string, object?>' in 'Model.Model(Dictionary<string, object?> property)' due to differences in the nullability of reference types.	JsonSerializerNullability	…\MyJsonSerializerContext.Model.g.cs	25	Active

Regression?

No response

Known Workarounds

No response

Configuration

net6.0 on version 6.0.0

Other information

No response

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Issue Details

---

Description

When the model type has a constructor with a generic type that has a nullable type argument, that nullability is not reproduced in the source-generated code for the JSON serializer and produces a nullability warning

Reproduction Steps

In a net6.0 classlib project:

using System.Text.Json.Serialization;

namespace JsonSerializerNullability;

public record Model
{
    public Model(Dictionary<string, object?> property) => Property = property;
    public Dictionary<string, object?> Property { get; }
}

[JsonSerializable(typeof(Model))]
internal partial class MyJsonSerializerContext : JsonSerializerContext
{ }

Expected behavior

No warnings should be produced

Actual behavior

Warning	CS8620	Argument of type 'Dictionary<string, object>' cannot be used for parameter 'property' of type 'Dictionary<string, object?>' in 'Model.Model(Dictionary<string, object?> property)' due to differences in the nullability of reference types.	JsonSerializerNullability	…\MyJsonSerializerContext.Model.g.cs	25	Active

Regression?

No response

Known Workarounds

No response

Configuration

net6.0 on version 6.0.0

Other information

No response

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

[eiriktsarpalis]

Related to #59464. This is actually related to nullability of generic parameters rather than constructor arguments. When working on a fix I thought I had added tests for nullable generic parameters

runtime/src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/RealWorldContextTests.cs

Lines 796 to 798 in 2b23bfa

	public GenericStruct<Uri?> NullableUriParameter { get; set; }
	public GenericStruct<int[]?> NullableArrayParameter { get; set; }
	public GenericStruct<MyPoco?> NullablePocoParameter { get; set; }

However it turns out this was never really being tested, since our test projects have their Nullable property set to annotations. We should service this.