What does #Task<T> indicate in an F# function signature?

In the following sample code:

namespace MyNamespace

open System.Threading.Tasks

module MyModule =

    let handler fn = task { return! fn () }

If I hover the handler function the signature is:
val handler: fn: (unit -> #Task<'b>) -> Task<'b>

I haven’t see the #Task before. What is the meaning of the # sign in this case?

>Solution :

#Task<'t> is short for 'a when 'a :> Task<'t> – that is, any subtype of Task<'t>