JSON source generator fails to compile if context type isn't in a namespace

[stephentoub]

This fails to compile:

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

class Program
{
    public static void Main() { }
}

internal class Book
{
    public string? Name { get; set; }
}

[JsonSerializable(typeof(Book))]
internal partial class MyJsonContext : JsonSerializerContext { }

with the errors:

Error	CS0534	'MyJsonContext' does not implement inherited abstract member 'JsonSerializerContext.GetTypeInfo(Type)'
Error	CS7036	There is no argument given that corresponds to the required formal parameter 'instanceOptions' of 'JsonSerializerContext.JsonSerializerContext(JsonSerializerOptions?, JsonSerializerOptions?)'

but wrap both the type to be serialized and the context type in a namespace, and then it compiles fine:

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

class Program
{
    public static void Main() { }
}

namespace Anything
{
    internal class Book
    {
        public string? Name { get; set; }
    }

    [JsonSerializable(typeof(Book))]
    internal partial class MyJsonContext : JsonSerializerContext { }
}

Tagging subscribers to this area: @eiriktsarpalis, @layomia
See info in area-owners.md if you want to be subscribed.

Issue Details

---

This fails to compile:

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

class Program
{
    public static void Main() { }
}

internal class Book
{
    public string? Name { get; set; }
}

[JsonSerializable(typeof(Book))]
internal partial class MyJsonContext : JsonSerializerContext { }

with the errors:

Error	CS0534	'MyJsonContext' does not implement inherited abstract member 'JsonSerializerContext.GetTypeInfo(Type)'
Error	CS7036	There is no argument given that corresponds to the required formal parameter 'instanceOptions' of 'JsonSerializerContext.JsonSerializerContext(JsonSerializerOptions?, JsonSerializerOptions?)'

but wrap both the type to be serialized and the context type in a namespace, and then it compiles fine:

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

class Program
{
    public static void Main() { }
}

namespace Anything
{
    internal class Book
    {
        public string? Name { get; set; }
    }

    [JsonSerializable(typeof(Book))]
    internal partial class MyJsonContext : JsonSerializerContext { }
}

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