Roslyn fails to detect nullability problems in C# 10 interpolated string handlers

[TessenR]

Version Used:

Branch main (26 Oct 2021)
Latest commit a4ec465 by Gen Lu:
Merge pull request #57338 from genlu/FixWarning

Fix 'AssemblyVersion overridden by auto-generated version' warning in build

Steps to Reproduce:

Compile and run the following code:

using System.Globalization;
using System.Runtime.CompilerServices;
using System.Text;

#nullable enable

string? s = null;
M(s, $"");

void M<T>(T t1, [InterpolatedStringHandlerArgument("t1")] CustomHandler c) {}

public partial struct CustomHandler
{
  public CustomHandler(int literalLength, int formattedCount, string t , out bool success) : this()
  {
    t.ToString();
      
    _builder = new();
    success = true;
  }
}

[System.Runtime.CompilerServices.InterpolatedStringHandler]
public partial struct CustomHandler
{
  private readonly StringBuilder _builder;
  public CustomHandler(int literalLength, int formattedCount, out bool success)
  {
    success = true;
    _builder = new();
  }
  public bool AppendLiteral(string literal)
  {
    _builder.AppendLine("literal:" + literal);
    return true;
  }
  public bool AppendFormatted(object o, int alignment = 0, string? format = null)
  {
    _builder.AppendLine("value:" + o?.ToString());
    _builder.AppendLine("alignment:" + alignment.ToString());
    _builder.AppendLine("format:" + format);
    return true;
  }
  public override string ToString() => _builder.ToString();
}

namespace System.Runtime.CompilerServices
{
  [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
  public sealed class InterpolatedStringHandlerAttribute : Attribute
  {
    public InterpolatedStringHandlerAttribute()
    {
    }
  }
}

public class CultureInfoNormalizer
{
  public static void Normalize()
  {
    CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
  }
}

namespace System.Runtime.CompilerServices
{
  [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
  public sealed class InterpolatedStringHandlerArgumentAttribute : Attribute
  {
    public InterpolatedStringHandlerArgumentAttribute(string argument) => Arguments = new string[] { argument };
    public InterpolatedStringHandlerArgumentAttribute(params string[] arguments) => Arguments = arguments;
    public string[] Arguments { get; }
  }
}

namespace System.Diagnostics.CodeAnalysis
{
  [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue)]
  public sealed class MaybeNullAttribute : Attribute
  {
  }
}

Expected Behavior:
Warning for a nulalble argument to the CustomHandler's non-nullable parameter string t

Actual Behavior:
No warnings at all in the program above. It crashes with a NullReferenceException at runtime

[333fred]

Duplicate of #54583.