NativeAOT returns null Type.Namespace for a nested generic type whose FullName includes a namespace #135106

Description

A nested generic type has a correct namespace-qualified FullName but a null Namespace under NativeAOT. The same executable under CoreCLR returns the namespace for both the constructed type and its generic definition.

This was found while validating generated Orleans serialization-contract registrations. A standalone reproduction without Orleans or other package dependencies reproduces it. Orleans' type-name formatter uses Type.Namespace, so a format/parse round-trip then loses the namespace. No formatter workaround was added.

Reproduction

A net10.0 console project with project-local <PublishAot>true</PublishAot>:

namespace NamespaceRepro;

internal static class Program
{
    private static void Main()
    {
        var type = typeof(Container.Hidden<int>);
        Console.WriteLine($"Dynamic code: {System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeSupported}");
        Console.WriteLine($"FullName: {type.FullName}");
        Console.WriteLine($"Namespace: {type.Namespace ?? "<null>"}");
        Console.WriteLine($"Definition namespace: {type.GetGenericTypeDefinition().Namespace ?? "<null>"}");
        Console.WriteLine($"Outer namespace: {typeof(Program).Namespace ?? "<null>"}");
    }

    internal static class Container
    {
        internal sealed class Hidden<T>;
    }
}

Publish and execute for win-x64. For comparison, run the same project with dotnet run -c Release -p:PublishAot=false.

Actual output

Native:

Dynamic code: False
FullName: NamespaceRepro.Program+Container+Hidden`1[[System.Int32, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]]
Namespace: <null>
Definition namespace: <null>
Outer namespace: NamespaceRepro

CoreCLR:

Dynamic code: True
FullName: NamespaceRepro.Program+Container+Hidden`1[[System.Int32, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]]
Namespace: NamespaceRepro
Definition namespace: NamespaceRepro
Outer namespace: NamespaceRepro

Environment

Windows x64, .NET SDK 10.0.401, runtime and Microsoft.DotNet.ILCompiler 10.0.12. Native publishing used Visual Studio 2022 environmental tools with IlcUseEnvironmentalTools=true. The standalone reproduction publishes normally without trim/AOT warning suppressions.

Expected behavior

The constructed nested type and its definition return NamespaceRepro, matching CoreCLR and their namespace-qualified FullName values. Related Orleans investigation: dotnet/orleans#11374.