support type parameter constraints

Close #165

Author: aspeddro

grammar.js

@@ -269,6 +269,7 @@ module.exports = grammar({
         choice('=', '+='),
         optional('private'),
         $._type,
+        repeat($.type_constraint),
         optional(seq('=', $._type)),
         repeat(alias($._type_declaration_and, $.type_declaration)),
       )),
@@ -322,6 +323,13 @@ module.exports = grammar({
       $.unit,
     ),
 
+    type_constraint: $ => seq(
+      'constraint',
+      $._type,
+      '=',
+      $._type
+    ),
+
     tuple_type: $ => prec.dynamic(-1, seq(
       '(',
       commaSep1t($._type),

test/corpus/type_declarations.txt

@@ -455,3 +455,34 @@ type t<-'a>
   (type_declaration
     (type_identifier)
     (type_parameters (type_identifier))))
+
+===========================================
+Type constraint
+===========================================
+
+type t<'a> = 'a constraint 'a = int
+type decorator<'a, 'b> = 'a => 'b constraint 'a = int constraint 'b = _ => _
+
+---
+
+(source_file
+  (type_declaration
+    (type_identifier)
+    (type_parameters (type_identifier))
+    (type_identifier)
+    (type_constraint
+      (type_identifier) (type_identifier)))
+
+  (type_declaration
+    (type_identifier)
+    (type_parameters (type_identifier) (type_identifier))
+    (function_type
+      (function_type_parameters (type_identifier))
+      (type_identifier))
+    (type_constraint
+      (type_identifier) (type_identifier))
+    (type_constraint
+      (type_identifier)
+      (function_type
+        (function_type_parameters (type_identifier))
+        (type_identifier)))))