ref and byref<> parameters in F# class members - f#

Question

I rewrote some my f# functions definitions to static members and stuck at ref/byref parameter error:
static member bar (a : byref<int>) = Foo.bar &a
Error FS0001 This expression was expected to have type
'int ref'
but here has type
'byref<'a>'
Are there some differences with byref parameters between 'let' and static definitions?
UPD:
It's worked example of what I have changed to static member definition:
> let rec foo (a :byref<int>) =
a <- a-1
if a > 0 then
System.Console.Write(a.ToString()); foo &a
else a
;;
val foo : a:byref<int> -> int
> let mutable a = 3;;
val mutable a : int = 3
> foo &a;;
21val it : int = 0
Now it looks like that and it doesn't work. Why? :
> type Foo() =
static member bar (a : byref<int>) =
a <- a-1
if a > 0 then
System.Console.Write(a.ToString()); foo &a
else a;;
type Foo =
class
new : unit -> Foo
static member bar : a:byref<int> -> int
end
> let mutable b = 3;;
val mutable b : int = 3
> Foo.bar &b;;
Foo.bar &b;;
--------^^
stdin(71,9): error FS0001: This expression was expected to have type
'int ref'
but here has type
'byref<'a>'

Answer 1

int ref is type definition of F# Reference Cell, while byref<'a> is an equivalent of C# ref/out arguments (an argument passed by reference even when it's a value type).
While it's impossible to tell anything more without reproducible snippet, it's quite possible that you've conflated two types:
let myFunc(a: int ref) = a := 1
// this is wrong
let a = 1
myFunc &a
// this is right
let a = ref 1
myFunc a