F#: explicit type parameters in operator binding

I'm trying to define operator with the explicit type parameters and constraints:

let inline (===)<'a, 'b
    when 'a : not struct
     and 'b : not struct> a b = obj.ReferenceEquals (a,b)

It works well in F# 2.0, but produces the:

warning FS1189:
Type parameters must be placed directly adjacent to the type name, e.g. "type C<'T>", not type "C <'T>"

So what is the right way to do explicit type arguments specification for operator definition?

p.s. Please don't tell me about implicit type parameters and some other workarounds, I want to solve concretely this issue.