Use Path.safe_relative and warn

lib/ex_unit/lib/ex_unit/filters.ex

@@ -45,7 +45,7 @@ defmodule ExUnit.Filters do
   end
 
   defp extract_line_numbers(file_path) do
-    case Path.relative_to_cwd(file_path) |> String.split(":") do
+    case normalize_relative_path(file_path) |> String.split(":") do
       [path] ->
         {path, []}
 
@@ -66,6 +66,17 @@ defmodule ExUnit.Filters do
     end
   end
 
+  defp normalize_relative_path(file_path) do
+    case Path.safe_relative(file_path) do
+      {:ok, path} ->
+        path
+
+      :error ->
+        IO.warn("invalid file path given as ExUnit filter: #{file_path}", [])
+        file_path
+    end
+  end
+
   defp to_line_number(str) do
     case Integer.parse(str) do
       {x, ""} when x > 0 -> x

lib/ex_unit/test/ex_unit/filters_test.exs

@@ -279,4 +279,24 @@ defmodule ExUnit.FiltersTest do
       assert output =~ "invalid line number given as ExUnit filter: -987"
     end
   end
+
+  test "file outside of the current directory" do
+    external_path = "../other_project/test/path.exs"
+
+    output =
+      ExUnit.CaptureIO.capture_io(:stderr, fn ->
+        assert ExUnit.Filters.parse_path(external_path) == {external_path, []}
+      end)
+
+    assert output =~ "invalid file path given as ExUnit filter: ../other_project/test/path.exs"
+
+    output =
+      ExUnit.CaptureIO.capture_io(:stderr, fn ->
+        assert ExUnit.Filters.parse_path("#{external_path}:123") ==
+                 {external_path,
+                  [{:exclude, [:test]}, {:include, [location: {external_path, 123}]}]}
+      end)
+
+    assert output =~ "invalid file path given as ExUnit filter: ../other_project/test/path.exs"
+  end
 end