Fix truthiness implementation (#15339)

Author: gldubc

lib/elixir/lib/module/types/descr.ex

@@ -1169,12 +1169,14 @@ defmodule Module.Types.Descr do
       :term ->
         :undefined
 
-      %{atom: {:union, set}}
-      when map_size(descr) == 1 and set in @false_or_nil_atoms ->
-        :always_false
+      %{atom: {:union, set}} when set in @false_or_nil_atoms ->
+        cond do
+          map_size(descr) == 1 -> :always_false
+          empty?(Map.delete(descr, :atom)) -> :always_false
+          true -> :undefined
+        end
 
-      %{atom: {:union, set}}
-      when map_size(descr) == 1 and not is_map_key(set, false) and not is_map_key(set, nil) ->
+      %{atom: {:union, set}} when not is_map_key(set, false) and not is_map_key(set, nil) ->
         :always_true
 
       %{atom: {:negation, %{nil => _, false => _}}} ->
@@ -1186,8 +1188,8 @@ defmodule Module.Types.Descr do
       _ when map_size(descr) == 0 ->
         :undefined
 
-      _ ->
-        :always_true
+      descr ->
+        if empty?(descr), do: :undefined, else: :always_true
     end
   end
 

lib/elixir/test/elixir/module/types/descr_test.exs

@@ -1505,6 +1505,20 @@ defmodule Module.Types.DescrTest do
         assert truthiness(type) == :always_true
         assert truthiness(dynamic(type)) == :always_true
       end
+
+      assert truthiness(union(atom([true]), integer())) == :always_true
+
+      empty_descr =
+        difference(tuple([number(), integer()]), open_tuple([float(), term()]))
+        |> difference(tuple([integer(), integer()]))
+
+      assert empty?(empty_descr)
+
+      assert truthiness(empty_descr) == :undefined
+      assert truthiness(dynamic(empty_descr)) == :undefined
+      assert truthiness(union(atom([nil]), empty_descr)) == :always_false
+
+      assert truthiness(union(atom([false]), empty_descr)) == :always_false
     end
 
     test "atom_fetch" do

lib/elixir/test/elixir/module/types/expr_test.exs

@@ -1596,6 +1596,34 @@ defmodule Module.Types.ExprTest do
                    # from: types_test.ex:LINE-1
                    y = 123
                """
+
+      assert typewarn!(
+               [x],
+               cond do
+                 if(x, do: true, else: 1) -> :if
+                 x -> :x
+               end
+             ) ==
+               {atom([:if, :x]),
+                ~l"""
+                this clause in cond will always match:
+
+                    if x do
+                      true
+                    else
+                      1
+                    end
+
+                since it has type:
+
+                    true or integer()
+
+                where "x" was given the type:
+
+                    # type: dynamic()
+                    # from: types_test.ex:LINE-7
+                    x
+                """}
     end
 
     test "Integer.to_string/1" do