MethodGroupNullable.csproj

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

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.2</TargetFramework>
    <LangVersion>8.0</LangVersion>

    <!-- for Visual Studio 16.2.x Preview -->
    <Nullable>enable</Nullable>

    <!-- for Visual Studio 2019, .NET Core SDK 3.0 Preview -->
    <NullableContextOptions>enable</NullableContextOptions>
  </PropertyGroup>

</Project>

Program.cs

using System;
using System.Linq;

namespace ConsoleApp1
{
    class Program
    {
        static bool X(Func<int, bool> predicate) => predicate(default);

        static void Main(string[] args)
        {
            X("abc".GenericExtensionMethod); // CS8622
            X(new Program().GenericExtensionMethod); // CS8622
            X(new object().GenericExtensionMethod); // No error
            X(Enumerable.Empty<int>().GenericExtensionMethod); // CS8622

            X(Enumerable.Empty<int>().Contains); // CS8622
        }
    }

    public static class Extension
    {
        public static bool GenericExtensionMethod<T>(this T instance, int value) => true;
    }
}