Custom Query Operators Fails at Runtime?

[Thorium]

I can extend QueryBuilder with custom operators, e.g.:

open System
open System.Linq
open Microsoft.FSharp.Linq

[<AutoOpen>]
module AsyncQueryBuilderOperations =

    type QueryBuilder with
        /// A query operator that returns the number of selected elements.
        [<CustomOperation("countAsync")>]
        member x.CountAsync(source:Linq.QuerySource<'T,'Q>) =
            source.Source.AsQueryable() |> getCountAsync

        /// A query operator that selects the first element from those selected so far.
        [<CustomOperation("headAsync")>]
        member x.HeadAsync(source:Linq.QuerySource<'T,'Q>) =
            source.Source.AsQueryable() |> getHeadAsync

        /// A query operator that selects the first element of those selected so far, or a default value if the sequence contains no elements.
        [<CustomOperation("headOrDefaultAsync")>]
        member x.HeadOrDefaultAsync(source:Linq.QuerySource<'T,'Q>) =
            async {
                let! res = source.Source.AsQueryable() |> getTryHeadAsync
                return match res with Some x -> x | None -> Unchecked.defaultof<'T>
            }

Then I can actually use them like:

[<Test >]
let ``simple select with countAsync``() =
    let dc = sql.GetDataContext()
    let query = 
        query {
            for cust in dc.Main.Customers do
            select cust.CustomerId
            countAsync
        } |> Async.RunSynchronously
    Assert.AreEqual(91, query)   

But the problem is that F# LINQ Query translator doesn't like them on runtime:

System.NotSupportedException : This is not a valid query expression. The 
method 'Microsoft.FSharp.Control.FSharpAsync`1[System.Int32] QueryBuilder.CountAsync
[String,Object](Microsoft.FSharp.Linq.QueryBuilder, Microsoft.FSharp.Linq.QuerySource`2
[System.String,System.Object])' was used in a query but is not recognized by the F#-to-LINQ query 
translator. Check the specification of permitted queries and consider moving some of the operations 
out of the query expression

at Microsoft.FSharp.Linq.QueryModule.TransInner(CanEliminate canElim, Boolean check, FSharpExpr immutQuery)
at Microsoft.FSharp.Linq.QueryModule.TransInnerAndCommit(CanEliminate canElim, Boolean check, FSharpExpr x)
at Microsoft.FSharp.Linq.QueryModule.TransInnerWithFinalConsume(CanEliminate canElim, FSharpExpr immutSource)
at Microsoft.FSharp.Linq.QueryModule.EvalNonNestedInner(CanEliminate canElim, FSharpExpr queryProducingSequence)
at Microsoft.FSharp.Linq.QueryModule.EvalNonNestedOuter(CanEliminate canElim, FSharpExpr tm)
at Microsoft.FSharp.Linq.QueryModule.clo@1737-1.Microsoft-FSharp-Linq-ForwardDeclarations-IQueryMethods-Execute[a,b](FSharpExpr`1 )
at QueryTests.simple select with countAsync() in C:\git\SQLProvider\tests\SqlProvider.Tests\QueryTests.fs:line 66

The problem is this file:

https://github.com/Microsoft/visualfsharp/blob/master/src/fsharp/FSharp.Core/Query.fs

raise ... unsupportedQueryConstruct

[dsyme]

@Thorium Could you resubmit using the new issue template please? thanks