Do not crash on nested bitstrings

Closes #14391.

Author: josevalim

lib/elixir/lib/module/types/pattern.ex

@@ -335,6 +335,10 @@ defmodule Module.Types.Pattern do
     {type, context}
   end
 
+  def of_match_var({:<<>>, _meta, args}, _expected, _expr, stack, context) do
+    {binary(), Of.binary(args, :match, stack, context)}
+  end
+
   def of_match_var(ast, expected, expr, stack, context) do
     of_match(ast, expected, expr, :default, stack, context)
   end

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

@@ -251,6 +251,10 @@ defmodule Module.Types.PatternTest do
       assert typecheck!([<<x::utf8>>], x) == integer()
     end
 
+    test "nested" do
+      assert typecheck!([<<0, <<x::bitstring>>::binary>>], x) == binary()
+    end
+
     test "error" do
       assert typeerror!([<<x::binary-size(2), x::float>>], x) == ~l"""
              incompatible types assigned to "x":