Fix compiler crash on invalid nested bitstring (#15280)

Author: lukaszsamson

lib/elixir/src/elixir_bitstring.erl

@@ -115,7 +115,9 @@ concat_or_prepend_bitstring(Meta, {'<<>>', PartsMeta, Parts} = ELeft, ERight, Ac
           [{'::', Meta, [ELeft, ERight]} | Acc]
       end;
     {bitstring, _, nil} ->
-      lists:reverse(Parts, Acc)
+      lists:reverse(Parts, Acc);
+    _ ->
+      [{'::', Meta, [ELeft, ERight]} | Acc]
   end;
 concat_or_prepend_bitstring(Meta, ELeft, ERight, Acc, _E, _RequireSize) ->
   [{'::', Meta, [ELeft, ERight]} | Acc].

lib/elixir/test/elixir/kernel/binary_test.exs

@@ -196,6 +196,12 @@ defmodule Kernel.BinaryTest do
     assert_compile_error(message, fn ->
       Code.eval_string(~s[<<"foo"::float>>])
     end)
+
+    assert_compile_error(message, fn ->
+      # We need to wrap the example below in a module because
+      # we attempt to continue compilation when inside a function
+      Code.compile_string("defmodule Repro do\n  def run, do:  <<(<<1>>)::integer>>\nend")
+    end)
   end
 
   @bitstring <<"foo", 16::4>>