Module import is considered unused if it used only for an extension method used in a CE

[Thecentury]

Repro steps Consider the following code:

open System.Threading.Tasks

module Builder =

    type AsyncBuilder with
        // allows to bind to the result of Tasks
        member _.Bind (task : Task<'a>, cont : 'a -> Async<'b>) =
            async.Bind (Async.AwaitTask task, cont)

open Builder

let task = Task.FromResult 1
let x = async {
    let! i = task
    return i + 1
}

Here the Builder module contains an extension method for an AsyncBuilder that enables the former to bind to the results of usual .Net tasks.

Expected behavior

No compiler warnings or errors.

Actual behavior

F# compiler considers this import to be not required: Warning "IDE0005: Open declaration can be removed".

But it is required as without it the compiler generates an error:

  X.fs(16, 14): [FS0001] This expression was expected to have type
    'Async<'a>'    
but here has type
    'Task<int>'

Related information

• Operating system — Windows 10
• .NET Framework 4.7.2
• Visual Studio 16.10.0