diff --git a/grammar.js b/grammar.js
index 6eccb08..48f284e 100644
--- a/grammar.js
+++ b/grammar.js
@@ -250,7 +250,7 @@ module.exports = grammar({
     ),
 
     _type_declaration: $ => seq(
-      $.type_identifier,
+      choice($.type_identifier, $.type_identifier_path),
       optional($.type_parameters),
       optional(seq(
         choice('=', '+='),
diff --git a/queries/highlights.scm b/queries/highlights.scm
index b65fc19..61f61f5 100644
--- a/queries/highlights.scm
+++ b/queries/highlights.scm
@@ -174,6 +174,7 @@
   "->"
   "|>"
   ":>"
+  "+="
   (uncurry)
 ] @operator
 
diff --git a/test/corpus/type_declarations.txt b/test/corpus/type_declarations.txt
index e5df8da..82a3b48 100644
--- a/test/corpus/type_declarations.txt
+++ b/test/corpus/type_declarations.txt
@@ -118,7 +118,7 @@ type t = ..
 
 type t += Other
 
-type t +=
+type M.t +=
   | Point
   | Line
 
@@ -133,7 +133,7 @@ type t +=
       (variant_declaration
         (variant_identifier))))
   (type_declaration
-    (type_identifier)
+    (type_identifier_path (module_identifier) (type_identifier))
     (variant_type
       (variant_declaration
         (variant_identifier))