ILC: can't resolve methods in generic types with generic type arguments as a parameter type in xml descriptor

[rolfbjarne]

Description

If an xml linker description tries to root a method from a generic type, and that method uses some of the generic type arguments in its signature, then ILC shows a warning during the build.

Reproduction Steps

C# code:

var obj = new MyGenericClass<string> ();
obj.GenericMethod ("hello");

public class MyGenericClass<T> {
	public void GenericMethod (T arg) {
	}
}

csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <TrimmerRootDescriptor Include="Roots.xml" />
  </ItemGroup>
</Project>

Roots.xml:

<linker>
  <assembly fullname="xml-generic-method">
    <type fullname="MyGenericClass`1">
      <!-- This signature uses the generic parameter name 'T', which ILC can't resolve -->
      <method signature="System.Void GenericMethod(T)" />
    </type>
  </assembly>
</linker>

Expected behavior

No warning, like building with ILLink does:

$ dotnet publish /p:PublishTrimmed=true
Restore complete (0.3s)
  xml-generic-method net10.0 osx-arm64 succeeded (4.2s) → bin/Release/net10.0/osx-arm64/publish/

Build succeeded in 4.7s

Actual behavior

Building with NativeAOT yields a warning:

$ dotnet publish /p:PublishAot=true
Restore complete (0.4s)
  xml-generic-method net10.0 osx-arm64 succeeded with 1 warning(s) (1.5s) → bin/Release/net10.0/osx-arm64/publish/
    /Users/rolf/test/dotnet/xml-generic-method/Roots.xml(5,8): warning IL2009: Could not find method 'System.Void GenericMethod(T)' on type 'MyGenericClass`1'.

Build succeeded with 1 warning(s) in 2.0s

Regression?

I don't think so.

Known Workarounds

Using the method name instead of the signature in the xml works:

<linker>
  <assembly fullname="xml-generic-method">
    <type fullname="MyGenericClass`1">
      <method name="GenericMethod" />
    </type>
  </assembly>
</linker>

but is less surgical and may root unrelated methods as well.

Configuration

$ dotnet --info

$ dotnet --info
.NET SDK:
 Version:           10.0.203
 Commit:            c23858a6d8
 Workload version:  10.0.200-manifests.333bc819
 MSBuild version:   18.3.3+c23858a6d

Runtime Environment:
 OS Name:     Mac OS X
 OS Version:  26.4
 OS Platform: Darwin
 RID:         osx-arm64
 Base Path:   /usr/local/share/dotnet/sdk/10.0.203/

.NET workloads installed:
 [macos]
   Installation Source: SDK 10.0.200
   Manifest Version:    26.4.10256/10.0.100
   Manifest Path:       /usr/local/share/dotnet/sdk-manifests/10.0.100/microsoft.net.sdk.macos/26.4.10256/WorkloadManifest.json
   Install Type:        FileBased

 [ios]
   Installation Source: SDK 10.0.200
   Manifest Version:    26.4.10256/10.0.100
   Manifest Path:       /usr/local/share/dotnet/sdk-manifests/10.0.100/microsoft.net.sdk.ios/26.4.10256/WorkloadManifest.json
   Install Type:        FileBased

 [maui]
   Installation Source: SDK 10.0.200
   Manifest Version:    10.0.20/10.0.100
   Manifest Path:       /usr/local/share/dotnet/sdk-manifests/10.0.100/microsoft.net.sdk.maui/10.0.20/WorkloadManifest.json
   Install Type:        FileBased

Configured to use loose manifests when installing new manifests.

Host:
  Version:      10.0.7
  Architecture: arm64
  Commit:       b16286c228

.NET SDKs installed:
  8.0.416 [/usr/local/share/dotnet/sdk]
  9.0.308 [/usr/local/share/dotnet/sdk]
  9.0.311 [/usr/local/share/dotnet/sdk]
  9.0.312 [/usr/local/share/dotnet/sdk]
  9.0.313 [/usr/local/share/dotnet/sdk]
  10.0.100 [/usr/local/share/dotnet/sdk]
  10.0.101 [/usr/local/share/dotnet/sdk]
  10.0.103 [/usr/local/share/dotnet/sdk]
  10.0.201 [/usr/local/share/dotnet/sdk]
  10.0.203 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 8.0.22 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.11 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.13 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.14 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.15 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.5 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.7 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 8.0.22 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.11 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.13 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.14 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.15 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.7 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
  x64   [/usr/local/share/dotnet/x64]
    registered at [/etc/dotnet/install_location_x64]

Environment variables:
  DOTNET_CLI_TELEMETRY_OPTOUT              [1]

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download

Other information

No response

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[MichalStrehovsky]

We could fix this to root the metadata for the method so that one can inspect custom attributes or parameters of the method. But this is of pretty limited value. To invoke the method or create a delegate to it this would have to be a closed instantiation so that we can generate machine code. ILLink.Descriptors.xml file format doesn't provide facilities to refer to a closed type/method. We intentionally didn't extend it because we want to get people off suppressing AOT warnings.

What is the E2E scenario this is needed for?

[rolfbjarne]

This is the method where we hit this:

https://github.com/dotnet/macios/blob/0d303817c2ac4b06db031717cb96f15e39ee8db0/src/UIKit/UIGestureRecognizer.cs#L101-L103

In the past, we've marked the Activated method in a custom trimmer step, but we're moving away from custom trimmer steps, so we've switched to generating an xml descriptor file, which we then pass to ILLink/ILC. And that's where we're running into this.

Note there will be code that creates a closed Callback<T> type somewhere, so what we'd need is to be able to call any corresponding closed Activated methods as well.

Going forward this will become unnecessary at some point, because we'll use trimmable type maps, and there will be a visible code path the trimmers will see and we won't need the xml descriptor file, but we're not quite there yet.

FWIW I haven't actually verified that the xml description is needed right now (ILC might already be generating all we need), we're just looking for [Preserve] attributes and trying to replicate the desired behavior using xml descriptions. That said, the behavior difference between ILLink and ILC seems unfortunate either way.

[MichalStrehovsky]

Going forward this will become unnecessary at some point, because we'll use trimmable type maps, and there will be a visible code path the trimmers will see and we won't need the xml descriptor file, but we're not quite there yet.

This is the most important part for me. Good. We don't even document ILLink.Descriptors.xml on learn anymore because that's how much we want people to stop suppressing trimming/AOT warnings and unconditionally rooting things.

I think copilot will be able to fix this bug.