Fix crash with @get on external of type unit => 'a (#7866)

* Fix crash with `@get` on external of type `unit => 'a`

* fix format

* CHANGELOG

Author: cknitt

CHANGELOG.md

@@ -23,6 +23,7 @@
 #### :bug: Bug fix
 
 - Fix code generation for emojis in polyvars and labels. https://github.com/rescript-lang/rescript/pull/7853
+- Fix crash with `@get` on external of type `unit => 'a`. https://github.com/rescript-lang/rescript/pull/7866
 
 #### :memo: Documentation
 

compiler/core/lam_compile_external_call.ml

@@ -421,7 +421,11 @@ let translate_ffi ?(transformed_jsx = false) (cxt : Lam_compile_context.t)
     | [obj] ->
       let obj = translate_scoped_access scopes obj in
       E.dot obj name
-    | _ -> assert false
+    | _ ->
+      (* This should have been caught in the frontend validation. *)
+      invalid_arg
+        "Internal compiler error: @get external called with wrong number of \
+         arguments. Expected exactly one object argument."
     (* Note these assertion happens in call site *))
   | Js_set {js_set_name = name; js_set_scopes = scopes} -> (
     (* assert (js_splice = false) ;  *)

compiler/frontend/ast_external_process.ml

@@ -899,7 +899,12 @@ let external_desc_of_non_obj (loc : Location.t) (st : external_desc)
    tagged_template = _;
   } ->
     if arg_type_specs_length = 1 then
-      Js_get {js_get_name = name; js_get_scopes = scopes}
+      (* Check if the first argument is unit, which is invalid for @get *)
+      match arg_type_specs with
+      | [{arg_type = Extern_unit}] ->
+        Location.raise_errorf ~loc
+          "Ill defined attribute %@get (unit argument is not allowed)"
+      | _ -> Js_get {js_get_name = name; js_get_scopes = scopes}
     else
       Location.raise_errorf ~loc
         "Ill defined attribute %@get (only one argument)"

tests/build_tests/super_errors/expected/get_unit_arg.res.expected

@@ -0,0 +1,12 @@
+
+  We've found a bug for you!
+  /.../fixtures/get_unit_arg.res:2:1-3:36
+
+  1 │ // Test case for issue #7676 - @get external with unit => 'a should give
+    │  proper error
+  2 │ @get
+  3 │ external foo: unit => string = "foo"
+  4 │ 
+  5 │ let x = foo()
+
+  Ill defined attribute @get (unit argument is not allowed)

tests/build_tests/super_errors/fixtures/get_unit_arg.res

@@ -0,0 +1,5 @@
+// Test case for issue #7676 - @get external with unit => 'a should give proper error
+@get
+external foo: unit => string = "foo"
+
+let x = foo()