Interceptors do not report nullability warnings when type parameter constraints are not satisfied

[TessenR]

Version Used:

Microsoft Visual Studio Professional 2022
Version 17.8.0 Preview 2.0
VisualStudio.17.Preview/17.8.0-pre.2.0+34112.27
Microsoft .NET Framework
Version 4.8.09037

Steps to Reproduce:

Compile the following code:

#nullable enable
using System.Runtime.CompilerServices;

partial class C
{
  public string? Method1<T>(T arg) => null;
}

static class Program
{
  public static void Main()
  {
    var c = new C();
    string? x = null;

    c.Method1(x);
  }
}

partial class C
{
  [InterceptsLocation("Program.cs", 16, 7)]
  public string? Interceptor1<T>(T arg) where T: notnull => arg.ToString();
}

namespace System.Runtime.CompilerServices
{
  [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
  public class InterceptsLocationAttribute : Attribute
  {
    public InterceptsLocationAttribute(string path, int line, int character) { }
  }
}

Expected Behavior:
A warning is reported for the intercepted call c.Method1(x) which is translated into C.Interceptor1<string?>(x); because the type argument doesn't satisfy the where T: notnull constraint on the interceptor method

Actual Behavior:
No warnings at all. The code crashes at runtime with a NullReferenceException

[jcouv update:] Relates to test plan #67421

[RikkiGibson]

The problem here is that the interceptor's type parameter constraints are checked during lowering. At that point, the nullable-reinferred version of the call has been discarded, and we are lowering the initially-bound version of the call.

roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Call.cs

Line 177 in 6648115

if (!interceptor.CheckConstraints(new ConstraintsHelper.CheckConstraintsArgs(this._compilation, this._compilation.Conversions, includeNullability: true, attributeLocation, this._diagnostics)))

It's not straightforward to solve, unfortunately, and I think I'd want to pursue some of the longer-term design questions before investing effort into solving it.