Arithmetic operations with GroupBy are not translated

[alexk8]

            var results = context.Orders
                .GroupBy(o =>  o.CustomerId)
                .Select(g => new
                {
                    CustomerId = g.Key,
                    Sum = g.Sum(o => o.Amount*o.Price),
                }).ToList();

Arithmetics are quite handy with "group by" but they are not supported (not translated to sql)
Error generated for warning 'Microsoft.EntityFrameworkCore.Query.QueryClientEvaluationWarning: The LINQ expression 'GroupBy([o].CustomerId, [o])' could not be translated and will be evaluated locally. '. This exception can be suppressed or logged by passing event ID 'RelationalEventId.QueryClientEvaluationWarning' to the 'ConfigureWarnings' method in 'DbContext.OnConfiguring' or 'AddDbContext'.
Tested on 2.1, 2.2, 3.0-preview5