This behavior doesn't seem strange to me, not compared to that function you want 
to define, but what the heck, here's one solution to get the type you want 

# let do_with_opt_conv ?(conv = Obj.magic) a = conv a;;
val do_with_opt_conv : ?conv:('a -> 'b) -> 'a -> 'b = <fun>

and here's a better one 

# let do_with_opt_conv ?(conv = fun x -> invalid_arg "do_with_opt_conv") a = conv a;;
val do_with_opt_conv : ?conv:('a -> 'b) -> 'a -> 'b = <fun>

-- Brian

On Tue, 2 Sep 2003, Christoph Bauer wrote:

> Hi,
> 
> ocamls type inference uses information of optionl arguments. This
> results in a strange behaviour. Here is an example:
> 
>         Objective Caml version 3.07+beta 2
> 
> # let do_with_conv ~conv a = conv a;;
> do_with_conv ~conv:int_of_string "1" ;;
> val do_with_conv : conv:('a -> 'b) -> 'a -> 'b = <fun>
> 
> This is ok.
> 
> # let do_with_opt_conv ?(conv = fun s -> s) a = conv a;;
> do_with_opt_conv ~conv:string_of_int "1";;- : int = 1
> # val do_with_opt_conv : ?conv:('a -> 'a) -> 'a -> 'a = <fun>
> # do_with_opt_conv ~conv:string_of_int "1";;
> 
> Is there a solution? It is not possible to 
> add the "right" type information:
> 
> # let do_with_opt_conv ?(conv : 'a -> 'b = fun s -> s) (a:'a) : 'b = conv a;;
> val do_with_opt_conv : ?conv:('a -> 'a) -> 'a -> 'a = <fun>
> 
> Thanks,
> Christoph Bauer
> 
> -- 
> proc self {} {foreach c [split [info body self] ""] d {14 -7 0 0 4 -67 4 73 11
> 69 24 -83 -15 6 -4 -84 78 20 11 -78 -1 -1 79 19 -8 4} { binary scan $c c c
> if {[catch {append r [format %c [expr $c+$d]]}]} {return $r};}};puts [self]
> 
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