Remove subsumed fix for #4862

[garrigue]

#4862 was fixed in 3.12 by using backtracking in Typecore.type_label_exp.
However, it seems that this has been useless for a long time now (checked that the example compiles fine in 4.07 without backtracking).
So let us remove this useless backtracking.
Also, properly call enforce_current_level when there are no variables, and remove a superfluous generalize_structure in type_exp/Pexp_let.

[gasche]

It's ironic that we spent a fair amount of effort discussing this backtracking code, which turns out to be unnecessary after all :-)

Two questions:

• Is the testcase of small bug in relaxed value restriction #4862 part of the testsuite?
• The problem with small bug in relaxed value restriction #4862 came from an expected_type annotation introducing a contravariant occurrence, which would then block generalization. Can you explain how the current implementation avoids this issue? (Is it still avoided if the user writes an explicit annotation, { f = (lazy (raise Not_found) : ('a list -> int) Lazy.t) }?)

[garrigue]

• The test is in typing-poly/poly.ml.
• { f = (lazy (raise Not_found) : ('a list -> int) Lazy.t) } has never been accepted, as named variables are not generalizable locally.
• maybe you meant { f = (lazy (raise Not_found) : (_ list -> int) Lazy.t) }, which still works after removal.

[gasche]

I'm still not sure why the example works when it did not work before, but I can tell that the code is vastly simpler so I'm all for it if it still passes the testsuite. Approved.

[garrigue]

Since the original fix was introduced in 3.12, before the addition of GADTs, a lot has changed since then. It is not too surprising that this kind of bug was actually fixed by other changes.