Converting from generic type to its constraint interface

I have created a generic class, with the constraint IComparable, as follows:

Public Class Foo(Of T As IComparable)
    Private _f As T

    Public Sub New(ByVal f As T)
        _f = f
    End Sub

    Public Function GetBar() As Bar
        Return New Bar(Me) 'compiler error: Value of type Foo(of T) 
                           'cannot be converted to Foo(of IComparable)
    End Function

End Class

I have another class which takes as a constructor argument an object of the first class defined:

Public Class Bar
    Private _foo As Foo(Of IComparable)

    Public Sub New(ByVal foo As Foo(Of IComparable))
        _foo = foo
    End Sub
End Class

However, I get the shown error when trying to create the Bar object.

Can anybody explain why am I getting this error? I would expect the compiler to know that type T is any IComparable type...

I have found a similar question here, but I don't know how to use it in my case.