Warn when equality is always disjoint

Author: josevalim

lib/elixir/lib/module/types/of.ex

@@ -364,6 +364,21 @@ defmodule Module.Types.Of do
     end
   end
 
+  def apply(:erlang, name, [left, right], expr, stack, context)
+      when name in [:==, :"/=", :"=:=", :"=/="] do
+    cond do
+      name in [:==, :"/="] and number_type?(left) and number_type?(right) ->
+        {boolean(), context}
+
+      disjoint?(left, right) ->
+        warning = {:mismatched_comparison, expr, context}
+        {boolean(), warn(__MODULE__, warning, elem(expr, 1), stack, context)}
+
+      true ->
+        {boolean(), context}
+    end
+  end
+
   def apply(mod, name, args, expr, stack, context) do
     case :elixir_rewrite.inline(mod, name, length(args)) do
       {mod, name} -> apply(mod, name, args, expr, stack, context)

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

@@ -511,6 +511,7 @@ defmodule Module.Types.ExprTest do
     test "works across numbers" do
       assert typecheck!([x = 123, y = 456.0], min(x, y)) == dynamic(union(integer(), float()))
       assert typecheck!([x = 123, y = 456.0], x < y) == boolean()
+      assert typecheck!([x = 123, y = 456.0], x == y) == boolean()
     end
 
     test "warns when comparison is constant" do
@@ -536,6 +537,29 @@ defmodule Module.Types.ExprTest do
                 While Elixir can compare across all types, you are comparing across types \
                 which are always distinct, and the result is either always true or always false
                 """}
+
+      assert typewarn!([x = 123, y = 456.0], x === y) ==
+               {boolean(),
+                ~l"""
+                comparison between incompatible types found:
+
+                    x === y
+
+                where "x" was given the type:
+
+                    # type: integer()
+                    # from: types_test.ex:LINE-2
+                    x = 123
+
+                where "y" was given the type:
+
+                    # type: float()
+                    # from: types_test.ex:LINE-2
+                    y = 456.0
+
+                While Elixir can compare across all types, you are comparing across types \
+                which are always distinct, and the result is either always true or always false
+                """}
     end
 
     test "warns on comparison with struct across dynamic call" do