Fix cond scope and improve coverage, closes #8542 (#8544)

Author: josevalim

lib/elixir/src/elixir_erl_pass.erl

@@ -109,16 +109,16 @@ translate({fn, Meta, Clauses}, S) ->
 %% Cond
 
 translate({'cond', CondMeta, [[{do, Clauses}]]}, S) ->
-  [{'->', Meta, [[Condition], Body]} = H | T] = lists:reverse(Clauses),
-
-  Case =
-    case Condition of
-      X when is_atom(X) and (X /= false) and (X /= nil) ->
-        build_cond_clauses(T, Body, Meta);
-      _ ->
-        Error = {{'.', Meta, [erlang, error]}, [], [cond_clause]},
-        build_cond_clauses([H | T], Error, Meta)
+  [{'->', Meta, [[Condition], _]} = H | T] = lists:reverse(Clauses),
+
+  FirstMeta =
+    if
+      is_atom(Condition), Condition /= false, Condition /= nil -> ?generated(Meta);
+      true -> Meta
     end,
+
+  Error = {{'.', Meta, [erlang, error]}, [], [cond_clause]},
+  Case = build_cond_clauses([H | T], Error, FirstMeta),
   translate(replace_case_meta(CondMeta, Case), S);
 
 %% Case

lib/elixir/test/elixir/fixtures/dialyzer/cond.ex

@@ -0,0 +1,27 @@
+defmodule Dialyzer.Cond do
+  def one_boolean do
+    cond do
+      true -> :ok
+    end
+  end
+
+  def two_boolean do
+    cond do
+      List.flatten([]) == [] -> :ok
+      true -> :ok
+    end
+  end
+
+  def one_otherwise do
+    cond do
+      :otherwise -> :ok
+    end
+  end
+
+  def two_otherwise do
+    cond do
+      List.flatten([]) == [] -> :ok
+      :otherwise -> :ok
+    end
+  end
+end

lib/elixir/test/elixir/kernel/dialyzer_test.exs

@@ -121,6 +121,11 @@ defmodule Kernel.DialyzerTest do
     assert_dialyze_no_warnings!(context)
   end
 
+  test "no warnings on cond", context do
+    copy_beam!(context, Dialyzer.Cond)
+    assert_dialyze_no_warnings!(context)
+  end
+
   test "no warnings on for comprehensions with bitstrings", context do
     copy_beam!(context, Dialyzer.ForBitstring)
     assert_dialyze_no_warnings!(context)

lib/elixir/test/elixir/kernel/special_forms_test.exs

@@ -4,4 +4,53 @@ defmodule Kernel.SpecialFormsTest do
   use ExUnit.Case, async: true
 
   doctest Kernel.SpecialForms
+
+  describe "cond" do
+    test "does not leak variables for one clause" do
+      x = 0
+
+      cond do
+        true ->
+          x = 1
+          x
+      end
+
+      assert x == 0
+    end
+
+    test "does not leak variables for one clause with non-boolean as catch-all" do
+      x = 0
+
+      cond do
+        :otherwise ->
+          x = 1
+          x
+      end
+
+      assert x == 0
+    end
+
+    test "does not leak variables for multiple clauses" do
+      x = 0
+
+      cond do
+        List.flatten([]) == [] ->
+          x = 1
+          x
+
+        true ->
+          x = 1
+          x
+      end
+
+      assert x == 0
+    end
+
+    test "does not warn on non-boolean as catch-all" do
+      cond do
+        List.flatten([]) == [] -> :good
+        :otherwise -> :also_good
+      end
+    end
+  end
 end