support non-parenthesized label colon type equal optional in type dec…

…larations

fixes reasonml#2056

formatTest/unit_tests/expected_output/syntax.re

@@ -1351,3 +1351,6 @@ foo(~a=?-1);
  */
 let z = {<state: 0, x: y>};
 let z = {<>};
+
+/* https://github.com/facebook/reason/issues/2056 */
+type foo = (~a: bool=?) => int;

formatTest/unit_tests/input/syntax.re

@@ -675,18 +675,18 @@ let myOptional           (~a=?,    ~b=?,      ()) = 10;
 type named =             (~a: int=?, ~b: int=?, unit) => int;
 /*F*/
 let optionalAlias        (~a as aa=?,  ~b as bb=?,  ()) = 10;
-/*G*/                       
+/*G*/
 let optionalAnnot        (~a as a:int =?, ~b as b:int=?, ()) = 10;
-/*H*/                       
+/*H*/
 let optionalAliasAnnot   (~a as aa:int =?, ~b as bb:int=?, ()) = 10;
-/*I: */                     
+/*I: */
 let defOptional          (~a as a=10, ~b as b=10, ()) = 10;
 type named =             (~a: int=?, ~b: int=?, unit) => int;
-/*J*/                       
+/*J*/
 let defOptionalAlias     (~a as aa=10, ~b as bb=10, ()) = 10;
-/*K*/                       
+/*K*/
 let defOptionalAnnot     (~a as a:int=10, ~b as b:int=10, ()) = 10;
-/*L*/                       
+/*L*/
 let defOptionalAliasAnnot(~a as aa:int=10, ~b as bb:int=10, ()) = 10;
 
 /*M: Invoking them - Punned */
@@ -1177,3 +1177,6 @@ foo(~a=?-1);
 */
 let z = {<state: 0, x: y>};
 let z = {<>};
+
+/* https://github.com/facebook/reason/issues/2056 */
+type foo = ~a:bool=? => int;

src/reason-parser/reason_parser.mly

@@ -4308,17 +4308,23 @@ unattributed_core_type:
     { $1 }
   | arrow_type_parameters EQUALGREATER core_type2
     { List.fold_right mktyp_arrow $1 $3 }
+  | as_loc(labelled_arrow_type_parameter_optional) EQUALGREATER core_type2
+    { mktyp_arrow ($1, false) $3 }
   | basic_core_type EQUALGREATER core_type2
     { mktyp (Ptyp_arrow (Nolabel, $1, $3)) }
 ;
 
+labelled_arrow_type_parameter_optional:
+  | TILDE LIDENT COLON core_type EQUAL optional
+    { ($6 $2, $4) }
+;
+
 arrow_type_parameter:
   | core_type
     { (Nolabel, $1) }
   | TILDE LIDENT COLON core_type
     { (Labelled $2, $4) }
-  | TILDE LIDENT COLON core_type EQUAL optional
-    { ($6 $2, $4) }
+  | labelled_arrow_type_parameter_optional { $1 }
 ;
 
 %inline uncurried_arrow_type_parameter: