Add (type _) as an allowed pattern

Previously a hole, `_`, was not allowed in place of the name of a `(type x)`
pattern.

parser.mly

@@ -381,16 +381,15 @@ bind :
 
 atpat :
   | head
-    { if $1.it = "_" then holeP@@at() else varP($1)@@at() }
+    { headP $1 }
   | LBRACE decon RBRACE
     { strP($2)@@at() }
   | LPAR RPAR
     { strP([])@@at() }
   | LPAR patlist RPAR
     { match $2 with [p] -> p | ps -> tupP(ps)@@at() }
-  | LPAR TYPE name typparamlist RPAR
-    { annotP(varP($3.it@@ati 3)@@ati 3,
-        funT($4, TypT@@ati 2, Pure@@ati 2)@@at())@@at() }
+  | LPAR TYPE head typparamlist RPAR
+    { annotP(headP $3, funT($4, TypT@@ati 2, Pure@@ati 2)@@at())@@at() }
 ;
 apppat :
   | atpat

regression.1ml

@@ -28,6 +28,10 @@ Equivalence: {
 
 ;;
 
+hole_is_allowed_type_pattern (type _ _) = ();
+
+;;
+
 type_error { type_error 101 };
 
 ;;

syntax.ml

@@ -251,6 +251,9 @@ let varP(x) = VarB(x, VarE("$"@@x.at)@@x.at)@@x.at, None
 let holeP : bind * typ option =
   EmptyB@@nowhere_region, None
 
+let headP id =
+  if id.it = "_" then holeP@@id.at else varP(id)@@id.at
+
 let asTopt(to1, to2) =
   match to1, to2 with
   | None, None -> None