Support generics in analyzer - #1854

Must still improve: won't work in the example when using Foo<Thing>, with Thing still an other object not properly marked with attributes, will improve it.

I didn't check the impact on the developer branch with v3, can look at it if requested

Thanks for contributing.
I'm glad you're already thinking of the impact on the develop branch. In fact I urge you to rebase now and target the develop branch initially, because that's where a bunch of this work is happening. We won't take the PR if it only works for master and creates merge conflicts in develop. But I'd take a PR that only targets develop.
After it works for develop, we can rebase the change back to master to see if it comes cleanly, and if it does, we can merge it there too. Or you can author the fix for both branches individually -- that works too. But develop is very much the priority right now.

Unfortunately I'm not sure to understand how analyzer are supposed to work on develop - I don't manage to have it running on sandbox project and am less familiar with SourceGenerator.
I didn't found anything in the migration markdown which explains it. For example this code

// Copyright (c) All contributors. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

using MessagePack;

namespace ConsoleApp1;

public class Program
{
    [MessagePackObject]
    public class A
    {
        public int Member { get; set; }
    }

    public static void Main(string[] args)
    {
        var bytes = MessagePackSerializer.Serialize(new A { Member = 10 });
        var b = MessagePackSerializer.Deserialize<A>(bytes);

        Console.WriteLine(b.Member == 10);
    }
}

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net8.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

    <ItemGroup>
      <ProjectReference Include="..\..\src\MessagePack.Analyzers\MessagePack.Analyzers.csproj"
                        OutputItemType="Analyzer"
                        ReferenceOutputAssembly="false" />
      <ProjectReference Include="..\..\src\MessagePack\MessagePack.csproj" />
    </ItemGroup>

</Project>

I don't get any warning/error at compilation for missing Key/IgnoreMember attributes, which would then crash at runtime.

Your last repro works fine on v3:

The repro in your original issue however may still be a problem. I'm writing a test now to see.

#1859 adds the test and fixes the bug for v3.
The fix doesn't readily port to v2, but I suspect logically it does if you want to update your PR with the fix I made for v3. Otherwise, let's close this PR.