Properly track variables inside comprehensions size, closes #15247

Author: josevalim

lib/elixir/src/elixir_erl_for.erl

@@ -318,8 +318,10 @@ no_var(ParentAnn, Elements) ->
 
 no_var_expr(Ann, {string, _, String}) -> [{var, Ann, '_'} || _ <- String];
 no_var_expr(Ann, _) -> [{var, Ann, '_'}].
-no_var_size({var, _, _}) -> throw(unbound_size);
-no_var_size(Size) -> Size.
+
+no_var_size(default) -> default;
+no_var_size(Size) when is_integer(Size) -> Size;
+no_var_size(_) -> throw(unbound_size).
 
 build_comprehension(Ann, Clauses, Expr, Into) ->
   {comprehension_kind(Into), Ann, Expr, comprehension_clause(Clauses)}.

lib/elixir/test/elixir/kernel/comprehension_test.exs

@@ -513,6 +513,11 @@ defmodule Kernel.ComprehensionTest do
     bin = <<8, 1, 32, 2, 3>>
     assert for(<<s, x::size(s) <- bin>>, into: "", do: <<x::size(s)>>) == <<1>>
     assert for(<<s, x::size(s) <- bin>>, into: %{}, do: {s, x}) == %{8 => 1}
+
+    # Computed
+    bin = <<1, 1, 2, 2, 3>>
+    assert for(<<s, x::size(s * 8) <- bin>>, into: "", do: <<x::size(s * 8)>>) == <<1, 2, 3>>
+    assert for(<<s, x::size(s * 8) <- bin>>, into: %{}, do: {s, x}) == %{1 => 1, 2 => 515}
   end
 
   test "binary for comprehensions where value is not used" do