add test for mel.uncurry

test/blackbox-tests/mel-uncurry-nesting.t

@@ -0,0 +1,58 @@
+Test the attribute @mel.uncurry at different level of nesting
+
+  $ . ./setup.sh
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (using melange 0.1)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target out)
+  >  (emit_stdlib false)
+  >  (preprocess (pps melange.ppx)))
+  > EOF
+
+Normal uncurry at first level works fine
+
+  $ cat > x.ml <<EOF
+  > external foo : ((unit -> unit)[@mel.uncurry]) -> unit
+  >   = "foo"
+  > EOF
+
+  $ dune build @melange
+
+Using `mel.uncurry` at 2nd level of callbacks raises some alerts
+
+  $ cat > x.ml <<EOF
+  > external foo :
+  >   (((unit -> unit)[@mel.uncurry]) -> (unit -> unit[@mel.uncurry])) -> unit
+  >   = "foo"
+  > EOF
+  $ dune build @melange
+  File "x.ml", line 2, characters 20-31:
+  2 |   (((unit -> unit)[@mel.uncurry]) -> (unit -> unit[@mel.uncurry])) -> unit
+                          ^^^^^^^^^^^
+  Alert unused: Unused attribute [@mel.uncurry]
+  This means such annotation is not annotated properly.
+  For example, some annotations are only meaningful in externals
+
+
+  File "x.ml", line 2, characters 52-63:
+  2 |   (((unit -> unit)[@mel.uncurry]) -> (unit -> unit[@mel.uncurry])) -> unit
+                                                          ^^^^^^^^^^^
+  Alert unused: Unused attribute [@mel.uncurry]
+  This means such annotation is not annotated properly.
+  For example, some annotations are only meaningful in externals
+
+
+
+In the case of uncurry nesting, we have to resort to the `[@u]` attribute
+
+  $ cat > x.ml <<EOF
+  > external foo : (((unit -> unit)[@u]) -> unit) -> unit = "foo"
+  > let () = foo (fun f -> f () [@u])
+  > EOF
+  $ dune build @melange
+