Fix wrong type information for constructors under type coercions

[voodoos]

Bug description:

Given the following file:

type t = U
type t' = U

let f : t  = U

The type hint given by Merlin for the U occuring in f is t' instead of t.
The same problem happens in patterns.

Solution:

• Refactor the Browse_raw.fold_node so that exp_extra nodes are considered children of their corresponding expression and not parents.
• Short the heuristic for type enclosings when the type can be directly extracted from the node.

[trefis]

Refactor the Browse_raw.fold_node so that exp_extra nodes are considered siblings of their corresponding expression and not parents.

You meant children right?

[trefis]

This looks good!
Although I wonder if the changes to the browse tree are actually necessary, have you tried without them? What diff does it produce on your tests if you remove them.

[voodoos]

If switching back to the original browse tree the wrong type is returned:

Done: 1608/1621 (jobs: 4)File "tests/test-dirs/type-enclosing/cons.t", line 1, characters 0-0:
         git (internal) (exit 1)
(cd _build/default && /usr/bin/git diff --no-index --color=always -u tests/test-dirs/type-enclosing/cons.t tests/test-dirs/type-enclosing/cons.t.corrected)
diff --git a/tests/test-dirs/type-enclosing/cons.t b/tests/test-dirs/type-enclosing/cons.t.corrected
index 2eb1f699..e04a6da2 100644
--- a/tests/test-dirs/type-enclosing/cons.t
+++ b/tests/test-dirs/type-enclosing/cons.t.corrected
@@ -13,7 +13,7 @@ Various parts of the cons.ml:
         "line": 4,
         "col": 14
       },
-      "type": "t",
+      "type": "t'",
       "tail": "no"
     },
     {
@@ -108,7 +108,7 @@ We aim to fix that in the future.
         "line": 15,
         "col": 15
       },
-      "type": "M.t",
+      "type": "M.u",
       "tail": "no"
     },
     {

[trefis]

👍

Do you want to clean-up the history or should I just squash everything?