ApiCompat does not notice breaking change in generic constraints

[KalleOlaviNiemitalo]

Describe the bug

ApiCompat does not notice if the right assembly breaks compatibility by having more constraints on a type parameter than the left assembly. See dotnet/runtime#99878.

To Reproduce

ConstraintApiCompat.csproj

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

  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;net8.0</TargetFrameworks>
    <LangVersion>7.3</LangVersion>
  </PropertyGroup>

</Project>

Class1.cs

namespace ConstraintApiCompat
{
    public sealed class Class1<TEnum>
#if !NETSTANDARD
	where TEnum : struct, System.Enum
#endif
    {
        // This is here just to show that ApiCompat works in other respects.
#if NETSTANDARD
	public void Extra() {}
#endif
    }
}

.config/dotnet-tools.json

{
  "version": 1,
  "isRoot": true,
  "tools": {
    "microsoft.dotnet.apicompat.tool": {
      "version": "8.0.203",
      "commands": [
        "apicompat"
      ]
    }
  }
}

Build

dotnet tool restore
dotnet build
dotnet apicompat --left=bin/Debug/netstandard2.0/ConstraintApiCompat.dll --right=bin/Debug/net8.0/ConstraintApiCompat.dll

Expected result

ApiCompat should report at least one API compatibility error about the type constraint:

API compatibility errors between 'bin/Debug/netstandard2.0/ConstraintApiCompat.dll' (left) and 'bin/Debug/net8.0/ConstraintApiCompat.dll' (right):
CPxxxx: Type 'ConstraintApiCompat.Class1<TEnum>' does not have constraints 'where TEnum : struct, System.Enum' on bin/Debug/netstandard2.0/ConstraintApiCompat.dll but has on bin/Debug/net8.0/ConstraintApiCompat.dll
CP0002: Member 'void ConstraintApiCompat.Class1<TEnum>.Extra()' exists on bin/Debug/netstandard2.0/ConstraintApiCompat.dll but not on bin/Debug/net8.0/ConstraintApiCompat.dll
API breaking changes found. If those are intentional, the APICompat suppression file can be updated by specifying the '--generate-suppression-file' parameter.

Actual result

ApiCompat does not report any API compatibility errors about the type constraint:

API compatibility errors between 'bin/Debug/netstandard2.0/ConstraintApiCompat.dll' (left) and 'bin/Debug/net8.0/ConstraintApiCompat.dll' (right):
CP0002: Member 'void ConstraintApiCompat.Class1<TEnum>.Extra()' exists on bin/Debug/netstandard2.0/ConstraintApiCompat.dll but not on bin/Debug/net8.0/ConstraintApiCompat.dll
API breaking changes found. If those are intentional, the APICompat suppression file can be updated by specifying the '--generate-suppression-file' parameter.

Exceptions (if any)

None.

Further technical details

.NET SDK:
 Version:           8.0.202
 Commit:            25674bb2f4
 Workload version:  8.0.200-manifests.a7f084b6

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19045
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\8.0.202\

.NET workloads installed:
 [android]
   Installation Source: SDK 8.0.200
   Manifest Version:    34.0.85/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.android\34.0.85\WorkloadManifest.json
   Install Type:              Msi

 [aspire]
   Installation Source: SDK 8.0.200
   Manifest Version:    8.0.0-preview.4.24156.9/8.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\8.0.100\microsoft.net.sdk.aspire\8.0.0-preview.4.24156.9\WorkloadManifest.json
   Install Type:              Msi


Host:
  Version:      8.0.3
  Architecture: x64
  Commit:       9f4b1f5d66

.NET SDKs installed:
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.526 [C:\Program Files\dotnet\sdk]
  6.0.420 [C:\Program Files\dotnet\sdk]
  8.0.103 [C:\Program Files\dotnet\sdk]
  8.0.202 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.28 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

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

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

No IDE.

[ericstj]

Looks like a gap - I don't see this tracked elsewhere. There was a sketch of support for this in the old API compat, but it wasn't enabled: https://github.com/dotnet/arcade/blob/release/6.0/src/Microsoft.DotNet.ApiCompat/src/Rules/Compat/CannotRemoveGenerics.cs

I'm taking a look at prototyping this.

[KalleOlaviNiemitalo]

Now that this has been fixed, I'm curious about whether the fix reveals any other mismatches in reference assemblies, similar to dotnet/runtime#99878. If there are any, will they show up in GitHub Checks for some "Update dependencies" pull request at dotnet/runtime?