OCaml: When the guard is in "fun"

If I read it correctly, the OCaml documentation ( here , in particular) says it fun

can have "when" guards, for example:

fun a b when a < b -> a + b;;

      

        
        
        
      

    

However, this generates a syntax error on "when". What gives? Am I reading this wrong?

Additional Information

The relevant parts from the documentation are copied below in the BNF type syntax:

expr ::= ...
       | fun multiple-matching
       | ...

multiple-matching ::= { pattern }+ [when expr] -> expr