Merge functions based on partiality rather than Parmatch.irrefutable

[lpw25]

translcore.ml merges together nested functions as long as the parameter is "fluid". This is what gives us a 2 parameter function for things like:

type r = R

let rr = fun R y -> 0

but the definition of fluid uses Parmatch.irrefutable which does not use type information. So things like:

type 'a t = T : int t | S : string t

let tt : int t -> _ -> _ = fun T y -> 0

still gives a function of 1 argument returning a function of 1 argument.

Since the partiality of the match (which does use type information) is already available this patch uses it in place of Parmatch.irrefutable. I think this is strictly more accurate and safe.

Note that there are optimizations in later passes (e.g. simplify) that also attempt to merge functions together. However, these can be quite fragile. For instance, the above case was not merged in 4.03, but this was accidentally fixed in 4.04 for ocamlopt by removing some debugging events. An example, which is not merged on trunk is bar below (unlike foo which is always merged):

type 'a foo = A of float * int | C of int * float;;

let foo = fun (A(_, x) | C(x, _)) y -> x + y;;

type 'a bar = A : float * int -> int bar | B : string bar | C : int * float -> int bar;;

let bar : int bar -> _ -> _ = fun (A(_, x) | C(x, _)) y -> x + y;;

[trefis]

Apart from what's mentioned in #1227 this seems like a strict improvement.

[trefis]

(of course it would be nice if @maranget could confirm)

[trefis]

I think this should be merged now that #1227 has been merged.

[gasche]

I reviewed the change (as part of #1308) and they seem correct.

[gasche]

(I don't understand why the CI check for Changes entry passes, but I will assume that this does not need a Change entry, add the label, and merge anyway.)