Do not crash for unknown var inside binary pattern in match (#6459)

Author: ggcampinho

lib/elixir/src/elixir_bitstring.erl

@@ -22,8 +22,8 @@ expand(BitstrMeta, Fun, [{'::', Meta, [Left, Right]} | T], Acc, E, RequireSize)
   %% on subparts, however we can't assign new variables.
   {ER, MatchSize} =
     case E of
-      {EExtracted, _} -> {EExtracted, false};          %% expand_arg,  no assigns
-      _               -> {E#{context := nil}, T /= []} %% expand, revert assigns
+      {EExtracted, _} -> {EExtracted, false}; %% expand_arg,  no assigns
+      _ -> {E#{context := nil, match_vars := warn}, T /= []} %% expand, revert assigns
     end,
 
   ERight = expand_specs(expr_type(ELeft), Meta, Right, ER, RequireSize or MatchSize),

lib/elixir/test/elixir/kernel/expansion_test.exs

@@ -1180,6 +1180,14 @@ defmodule Kernel.ExpansionTest do
     assert_raise CompileError, ~r"unhandled operator ->", fn ->
       expand(quote do: (foo -> bar))
     end
+
+    assert_raise CompileError,
+      ~r"size in bitstring expects an integer or a variable as argument, got: foo\(\)",
+      fn ->
+        expand(quote do
+          fn <<_::size(foo)>> -> :ok end
+        end)
+      end
   end
 
   ## Helpers