Fix Integer.undigits/2 to validate negative out-of-range digits (#15160)

The validation was asymmetric - it rejected positive digits >= base
but silently accepted negative digits <= -base. This fix adds the
missing check for negative out-of-range digits.

Author: pnezis

lib/elixir/lib/integer.ex

@@ -297,8 +297,9 @@ defmodule Integer do
 
   defp undigits([], _base, acc), do: acc
 
-  defp undigits([digit | _], base, _) when is_integer(digit) and digit >= base,
-    do: raise(ArgumentError, "invalid digit #{digit} in base #{base}")
+  defp undigits([digit | _], base, _)
+       when is_integer(digit) and (digit >= base or digit <= -base),
+       do: raise(ArgumentError, "invalid digit #{digit} in base #{base}")
 
   defp undigits([digit | tail], base, acc) when is_integer(digit),
     do: undigits(tail, base, acc * base + digit)

lib/elixir/test/elixir/integer_test.exs

@@ -144,6 +144,14 @@ defmodule IntegerTest do
     assert_raise ArgumentError, "invalid digit 17 in base 16", fn ->
       Integer.undigits([1, 2, 17], 16)
     end
+
+    assert_raise ArgumentError, "invalid digit -10 in base 10", fn ->
+      Integer.undigits([-10], 10)
+    end
+
+    assert_raise ArgumentError, "invalid digit -17 in base 16", fn ->
+      Integer.undigits([1, 2, -17], 16)
+    end
   end
 
   test "parse/2" do