Ash.CustomExpression compilation error, anonymous function not allowed

Describe the bug Ash.CustomExpression with a passed function raises compile time error

== Compilation error in file lib/expressions/levenshtein_distance.ex ==
** (RuntimeError) The only kind of anonymous functions allowed in expressions are in the format `&Module.function/arity`.

Got: &__MODULE__.levenshtein/2

    (ash 3.4.39) lib/ash/expr/expr.ex:763: Ash.Expr.do_expr/2
    (elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2
    (ash 3.4.39) lib/ash/expr/expr.ex:793: Ash.Expr.do_expr/2
    (ash 3.4.39) expanding macro: Ash.Expr.expr/1
    lib/expressions/levenshtein_distance.ex:21: Domain.Expressions.LevenshteinDistance.expression/2

To Reproduce

implement lib/expressions/levenshtein_distance.ex described in the docs for Ash.CustomExpression

defmodule Domain.Expressions.LevenshteinDistance do
use Ash.CustomExpression,
name: :levenshtein,
arguments: [
  [:string, :string]
]

def expression(AshPostgres.DataLayer, [left, right]) do
{:ok, expr(fragment("levenshtein(?, ?)", left, right))}
end

# It is good practice to always define an expression for `Ash.DataLayer.Simple`,
# as that is what Ash will use to run your custom expression in Elixir.
# This allows us to completely avoid communicating with the database in some cases.

def expression(data_layer, [left, right])
  when data_layer in [
         Ash.DataLayer.Ets,
         Ash.DataLayer.Simple
       ] do
{:ok, expr(fragment(&__MODULE__.levenshtein/2, left, right))}
end

# always define this fallback clause as well
def expression(_data_layer, _args), do: :unknown

@doc "Computes the levenshtein distance of two strings"
def levenshtein(left, right) do
# ......
end
end

== Compilation error in file lib/expressions/levenshtein_distance.ex ==
** (RuntimeError) The only kind of anonymous functions allowed in expressions are in the format `&Module.function/arity`.

Got: &__MODULE__.levenshtein/2

    (ash 3.4.39) lib/ash/expr/expr.ex:763: Ash.Expr.do_expr/2
    (elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2
    (ash 3.4.39) lib/ash/expr/expr.ex:793: Ash.Expr.do_expr/2
    (ash 3.4.39) expanding macro: Ash.Expr.expr/1
    lib/expressions/levenshtein_distance.ex:21: Domain.Expressions.LevenshteinDistance.expression/2

Add to config/config.exs

config :ash, :custom_expressions, [Domain.Expressions.LevenshteinDistance]

Bonus round; use the absolute module name instead of __MODULE__

def expression(data_layer, [left, right])
  when data_layer in [
         Ash.DataLayer.Ets,
         Ash.DataLayer.Simple
       ] do
{:ok, expr(fragment(&Domain.Expressions.LevenshteinDistance.levenshtein/2, left, right))}
end


== Compilation error in file lib/expressions/levenshtein_distance.ex ==
** (RuntimeError) The only kind of anonymous functions allowed in expressions are in the format `&Module.function/arity`.

Got: &Domain.Expressions.LevenshteinDistance.levenshtein/2

(ash 3.4.39) lib/ash/expr/expr.ex:763: Ash.Expr.do_expr/2
(elixir 1.17.2) lib/enum.ex:1703: Enum."-map/2-lists^map/1-1-"/2
(ash 3.4.39) lib/ash/expr/expr.ex:793: Ash.Expr.do_expr/2
(ash 3.4.39) expanding macro: Ash.Expr.expr/1
lib/expressions/levenshtein_distance.ex:21: Domain.Expressions.LevenshteinDistance.expression/2



**Expected behavior**
The custom expression should compile and the expression should be usable in a resource.

**Runtime**
 - Elixir version
   1.17.2
 - Erlang version
   Erlang/OTP 27 [erts-15.0.1]
 - OS
   Ubuntu 22.04.5 LTS
 - Ash version
   3.4.39
 - any related extension versions
   ash_postgres: 2.4.12

ash-project / ash

Ash.CustomExpression compilation error, anonymous function not allowed #1571