Aug 16, 2025 · Aug 19, 2025 · Sep 10, 2025 · Sep 11, 2025 · Sep 10, 2025
diff --git a/compiler/ml/typetexp.ml b/compiler/ml/typetexp.ml
           pack_txt = p;
         })
      ty
  | Ptyp_extension ext ->
    raise (Error_forward (Builtin_attributes.error_of_extension ext))
  | Ptyp_extension ext -> (
    match ext with
    | ({txt = "typeof"; loc = ext_loc}, payload) -> (
      (* %typeof payload must be a single identifier *)
      match Ast_payload.as_ident payload with
      | Some ({txt = lid; loc = lid_loc} as _ident) -> (
        (* Lookup the value and embed a generic instance of its type.
           Using a generic instance avoids capturing weak (non-generalized)
           type variables from the value into a type position. *)
        let (_path, desc) = find_value env lid_loc lid in
        let ty = Ctype.generic_instance env desc.val_type in
        (* Build a core_type node carrying the looked up type; we mark the
           desc as any since downstream only consults ctyp_type for typing. *)
        ctyp Ttyp_any ty)
      | None ->
        let msg =
          "%%typeof expects an identifier. Example: type t = %typeof(x)"
        in
        raise (Error_forward (Location.error ~loc:ext_loc msg)))
    | _ -> raise (Error_forward (Builtin_attributes.error_of_extension ext)))

 and transl_poly_type env policy t =
  transl_type env policy (Ast_helper.Typ.force_poly t)
diff --git a/tests/analysis_tests/tests/src/Hover.res b/tests/analysis_tests/tests/src/Hover.res

 type aliased = variant
 //    ^hov

 let myFn = (a, b) => a ++ b->Int.toString

 type fnType = %typeof(myFn)
 //    ^hov
diff --git a/tests/analysis_tests/tests/src/expected/Hover.res.txt b/tests/analysis_tests/tests/src/expected/Hover.res.txt
 Hover src/Hover.res 281:6
 {"contents": {"kind": "markdown", "value": "```rescript\ntype aliased = variant\n```\n\n---\n\n```\n \n```\n```rescript\ntype variant = CoolVariant | OtherCoolVariant\n```\nGo to: [Type definition](command:rescript-vscode.go_to_location?%5B%22Hover.res%22%2C251%2C0%5D)\n"}}

 Hover src/Hover.res 286:6
 {"contents": {"kind": "markdown", "value": "```rescript\ntype fnType = (string, int) => string\n```"}}

diff --git a/tests/build_tests/super_errors/expected/typeof_mismatch.res.expected b/tests/build_tests/super_errors/expected/typeof_mismatch.res.expected

  [1;31mWe've found a bug for you![0m
  [36m/.../fixtures/typeof_mismatch.res[0m:[2m6:28[0m

  4 [2m│[0m
  5 [2m│[0m let f: fnType = myFn
  [1;31m6[0m [2m│[0m let ff: fnType = (a, b) => [1;31ma[0m->Int.toString + b

  This has type: [1;31mstring[0m
  But this function argument is expecting: [1;33mint[0m

  You can convert [1;33mstring[0m to [1;33mint[0m with [1;33mInt.fromString[0m.
diff --git a/tests/build_tests/super_errors/fixtures/typeof_mismatch.res b/tests/build_tests/super_errors/fixtures/typeof_mismatch.res
 let myFn = (a, b) => a ++ b->Int.toString

 type fnType = %typeof(myFn)

 let f: fnType = myFn
 let ff: fnType = (a, b) => a->Int.toString + b
diff --git a/tests/tests/src/Typeof.res b/tests/tests/src/Typeof.res
 let myFn = (a, b) => a ++ b->Int.toString

 type fnType = %typeof(myFn)

 let f: fnType = myFn
Original file line number	Diff line number	Diff line change
Expand Up		@@ -565,8 +565,26 @@ and transl_type_aux env policy styp =
		pack_txt = p;
		})
		ty
		\| Ptyp_extension ext ->
		raise (Error_forward (Builtin_attributes.error_of_extension ext))
		\| Ptyp_extension ext -> (
		match ext with
		\| ({txt = "typeof"; loc = ext_loc}, payload) -> (
		(* %typeof payload must be a single identifier *)
		match Ast_payload.as_ident payload with
		\| Some ({txt = lid; loc = lid_loc} as _ident) -> (
		(* Lookup the value and embed a generic instance of its type.
		Using a generic instance avoids capturing weak (non-generalized)
		type variables from the value into a type position. *)
		let (_path, desc) = find_value env lid_loc lid in
		let ty = Ctype.generic_instance env desc.val_type in
		(* Build a core_type node carrying the looked up type; we mark the
		desc as any since downstream only consults ctyp_type for typing. *)
		ctyp Ttyp_any ty)
		\| None ->
		let msg =
		"%%typeof expects an identifier. Example: type t = %typeof(x)"
		in
		raise (Error_forward (Location.error ~loc:ext_loc msg)))
		\| _ -> raise (Error_forward (Builtin_attributes.error_of_extension ext)))

		and transl_poly_type env policy t =
		transl_type env policy (Ast_helper.Typ.force_poly t)
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -281,3 +281,8 @@ module Arr = Belt.Array

		type aliased = variant
		// ^hov

		let myFn = (a, b) => a ++ b->Int.toString

		type fnType = %typeof(myFn)
		// ^hov
Original file line number	Diff line number	Diff line change
Expand Up		@@ -348,3 +348,6 @@ Hover src/Hover.res 278:8
		Hover src/Hover.res 281:6
		{"contents": {"kind": "markdown", "value": "```rescript\ntype aliased = variant\n```\n\n---\n\n```\n \n```\n```rescript\ntype variant = CoolVariant \| OtherCoolVariant\n```\nGo to: [Type definition](command:rescript-vscode.go_to_location?%5B%22Hover.res%22%2C251%2C0%5D)\n"}}

		Hover src/Hover.res 286:6
		{"contents": {"kind": "markdown", "value": "```rescript\ntype fnType = (string, int) => string\n```"}}
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,12 @@

		[1;31mWe've found a bug for you![0m
		[36m/.../fixtures/typeof_mismatch.res[0m:[2m6:28[0m

		4 [2m│[0m
		5 [2m│[0m let f: fnType = myFn
		[1;31m6[0m [2m│[0m let ff: fnType = (a, b) => [1;31ma[0m->Int.toString + b

		This has type: [1;31mstring[0m
		But this function argument is expecting: [1;33mint[0m

		You can convert [1;33mstring[0m to [1;33mint[0m with [1;33mInt.fromString[0m.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,6 @@
		let myFn = (a, b) => a ++ b->Int.toString

		type fnType = %typeof(myFn)

		let f: fnType = myFn
		let ff: fnType = (a, b) => a->Int.toString + b
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,5 @@
		let myFn = (a, b) => a ++ b->Int.toString

		type fnType = %typeof(myFn)

		let f: fnType = myFn