add simple error message indicating dict syntax

Author: zth

compiler/ml/error_message_utils.ml

@@ -223,6 +223,10 @@ let print_extra_type_clash_help ~extract_concrete_typedecl ~env loc ppf
       \  - Use a tuple, if your array is of fixed length. Tuples can mix types \
        freely, and compiles to a JavaScript array. Example of a tuple: `let \
        myTuple = (10, \"hello\", 15.5, true)"
+  | _, Some (_, {desc = Tconstr (p2, _, _)}) when Path.same Predef.path_dict p2
+    ->
+    fprintf ppf
+      "@,@,Dicts are written like: @{<info>dict{\"a\": 1, \"b\": 2}@}@,"
   | _, Some ({Types.desc = Tconstr (_p1, _, _)}, {desc = Tconstr (p2, _, _)})
     when Path.same Predef.path_unit p2 ->
     fprintf ppf

tests/build_tests/super_errors/expected/dict_helper.res.expected

@@ -0,0 +1,13 @@
+
+  We've found a bug for you!
+  /.../fixtures/dict_helper.res:3:11-23
+
+  1 │ external takesDict: dict<string> => unit = "takesDict"
+  2 │ 
+  3 │ takesDict({"test": "1"})
+  4 │ 
+
+  This has type: {"test": string}
+  But it's expected to have type: dict<string>
+  
+  Dicts are written like: dict{"a": 1, "b": 2}

tests/build_tests/super_errors/fixtures/dict_helper.res

@@ -0,0 +1,3 @@
+external takesDict: dict<string> => unit = "takesDict"
+
+takesDict({"test": "1"})