Mix test parameters into the per-test :rand seed (#15869)

ExUnit seeds each test process with {phash2(module), phash2(name), seed}.
parameterize re-runs the module per parameter set without changing the
test name, so every parameter set of a test started from the same :rand
state and drew the same random sequence.

Hash the parameters together with the name when they are present. Tests
without parameters keep their previous seed, so existing --seed
reproductions are unaffected.

Assisted-by: Claude Code:claude-fable-5-1

Signed-off-by: Jechol Lee <mr.jechol@gmail.com>

Author: jechol

lib/ex_unit/lib/ex_unit/runner.ex

@@ -550,7 +550,12 @@ defmodule ExUnit.Runner do
 
   ## Helpers
 
-  defp generate_test_seed(seed, %ExUnit.Test{module: module, name: name}, rand_algorithm) do
+  # Parameterized tests share module and name, so mix the parameters into the seed
+  # to give each parameter set its own random sequence. Tests without parameters keep
+  # the seed they had before, so existing `--seed` reproductions are unaffected.
+  defp generate_test_seed(seed, %ExUnit.Test{} = test, rand_algorithm) do
+    %ExUnit.Test{module: module, name: name, parameters: parameters} = test
+    name = if parameters == %{}, do: name, else: {name, parameters}
     :rand.seed(rand_algorithm, {:erlang.phash2(module), :erlang.phash2(name), seed})
   end
 

lib/ex_unit/test/ex_unit_test.exs

@@ -717,6 +717,27 @@ defmodule ExUnitTest do
     assert_receive {:tmp_dir, tmp_dir2} when tmp_dir1 != tmp_dir2
   end
 
+  test "parameterized tests get a different seed for each parameter set" do
+    Process.register(self(), :parameterized_seed_tests)
+
+    defmodule ParameterizedSeedTests do
+      use ExUnit.Case, async: true, parameterize: [%{value: 1}, %{value: 2}]
+
+      test "hello world" do
+        send(:parameterized_seed_tests, {:rand, :rand.uniform(1_000_000)})
+      end
+    end
+
+    configure_and_reload_on_exit(seed: 1)
+
+    capture_io(fn ->
+      assert ExUnit.run() == %{failures: 0, skipped: 0, total: 2, excluded: 0}
+    end)
+
+    assert_receive {:rand, rand1}
+    assert_receive {:rand, rand2} when rand1 != rand2
+  end
+
   test "empty parameterized tests" do
     defmodule EmptyParameterizedTests do
       use ExUnit.Case, async: true, parameterize: []