Warn when no service to replace

Start with the ReplaceService<SqlServerTypeMapper, CustomSqlServerTypeMapper>() call and reproduce the behavior using the supplied ReplaceTypeMapperOk test on EF Core 2.0. Trace the service-replacement path to determine why the custom mapper is not invoked; done means the issue is diagnosed and the requested warning is covered by a regression test.

I'm trying to create an example of replacing a service inside EF Core as shown in a Channel 9 video by Rowan Miller starting at 45 minutes in.

My example (see below) doesn't work. And I can confirm that the constructor is not called, nor the overriden method are called, as my break points are never triggered. Can you tell me what I am doing wrong.

Steps to reproduce

Here is one of the versions I have tried (I have tried a few permutations and none work).

Here is the unit test, with the new CustomSqlServerTypeMapper

public class CustomSqlServerTypeMapper : SqlServerTypeMapper
{
    public CustomSqlServerTypeMapper(RelationalTypeMapperDependencies dependencies) 
        : base(dependencies) {}

    public override RelationalTypeMapping FindMapping(IProperty property)
    {
        var currentMapping = base.FindMapping(property);
        if (property.ClrType == typeof(string) && property.Name.EndsWith("Ascii"))
            return new StringTypeMapping(currentMapping.StoreType, 
                currentMapping.DbType, true, currentMapping.Size);

        return currentMapping;
    }

    public override RelationalTypeMapping FindMapping(string storeType)
    {
        return base.FindMapping(storeType);
    }
}

[Fact]
public void ReplaceTypeMapperOk()
{
    //SETUP
    var connectionString = AppConstants.ConnectionString;
    var optionsBuilder = new DbContextOptionsBuilder<TestDbContext>();
    optionsBuilder.UseSqlServer(connectionString);
    optionsBuilder.ReplaceService<SqlServerTypeMapper, CustomSqlServerTypeMapper>();

    using (var context = new TestDbContext(optionsBuilder.Options))
    {
        context.Database.EnsureDeleted();
        context.Database.EnsureCreated();

        //ATTEMPT 
        var entity = context.Model.FindEntityType(typeof(ScalarEntity));
        var mapInfo1 = entity.GetProperty(nameof(ScalarEntity.StringAscii));

        //VERIFY
        mapInfo1.ClrType.ShouldEqual(typeof(string));
        mapInfo1.IsUnicode().ShouldEqual(false);

    }
}

My entity class is

public class ScalarEntity
{
    public int Id { get; set; }

    public string StringMax { get; set; }

    [Required]
    [MaxLength(20)]
    public string String20 { get; set; }

    public string StringAscii { get; set; }
}

Further technical details

EF Core version: 2.0.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10
IDE: VS2017 15.3