Array(n).fill(value) isn't typechecked

[julienw]

With this code, Flow doesn't find the error:

type Plop = {
  prop: string[],
};

var a: Plop = {
  prop: Array(5).fill(1),
}

I believe this is because the Array constructor returns an array of any items in

flow/lib/core.js

Line 293 in f796b60

static (...values:Array<any>): Array<any>;

.

try link

[julienw]

Interestingly it works fine with new Array :)

[canova]

ECMA says Array function is the same with the array constructor. I don't know the codebase but probably we are missing that function or there is something with its type.
See the spec (section 15.4.1, The Array Constructor Called as a Function)

[goodmind]

Array constructor is defined in OCaml code for some reason

[goodmind]

flow/src/typing/statement.ml

Lines 3068 to 3115 in f796b60

	| New {
	New.callee = callee_loc, Identifier (id_loc, ({ Ast.Identifier.name= "Array" as n; comments= _ } as name));
	targs;
	arguments
	} -> (
	let targts = Option.map targs (fun (loc, args) ->
	loc, (convert_tparam_instantiations cx SMap.empty args)
	) in
	let args = Core_list.map ~f:(expression_or_spread cx) arguments in
	let result = match targts, args with
	| Some (loc, ([t], [elem_t])), [Arg argt, arg] -> Ok (Some (loc, elem_t, t), argt, arg)
	| None, [Arg argt, arg] -> Ok (None, argt, arg)
	| None, _ -> Error (Error_message.EUseArrayLiteral loc)
	| Some _, _ ->
	Error Error_message.(ECallTypeArity {
	call_loc = loc;
	is_new = true;
	reason_arity = Reason.(locationless_reason (RType n));
	expected_arity = 1;
	})
	in
	match result with
	| Ok (targ_t, arg_t, arg) ->
	let reason = mk_reason (RCustom "new Array(..)") loc in
	let length_reason =
	replace_reason_const (RCustom "array length") reason in
	Flow.flow_t cx (arg_t, DefT (length_reason, bogus_trust (), NumT AnyLiteral));
	let t, targs = match targ_t with
	| Some (loc, ast, ExplicitArg t) -> t, Some (loc, [ast])
	| Some (_, _, ImplicitArg _)
	| None ->
	let element_reason =
	replace_reason_const (RCustom "array element") reason in
	Tvar.mk cx element_reason, None
	in
	let id_t = identifier cx name callee_loc in
	(* TODO - tuple_types could be undefined x N if given a literal *)
	(loc, DefT (reason, bogus_trust (), ArrT (ArrayAT (t, None)))),
	New { New.
	callee = (callee_loc, id_t), Identifier ((id_loc, id_t), name);
	targs;
	arguments = [arg];
	}
	| Error err ->
	Flow.add_output cx err;
	Tast_utils.error_mapper#expression ex
	)

[goodmind]

Not sure why it's not possible to typecheck it like this instead of doing this kind of hardcoding
https://flow.org/try/#0CYUwxgNghgTiAEkoGdnwLIE8CCMZUwB4AVAPngG8AoeRAewDtkAXGAVzGbpgAoGAueAzYBbAEYgYASkG58RECIAOzTKQDcNek1YcuvAHRHYAc2SDiAbQC6M+HIIkNWrSyjMAlmHh9Bw8ZJ2DgrKqs60bp7ePEYGpubwVrayeI5kmgC+LmCMLPAAZnR08AC8GDipmDwArFJUOTrwYrCl5cE8AIwANPAATD0AzHX1ucwFRQAMrQwgAO5tlTV1DXnNMFNlM-NY7d19g1JAA
this error just doesn't make sense

13: const bar0 = new Array(1, 2, 3)                 
^ Use an array literal instead of `new Array(...)`.