Raise when splicing cons without head, closes #15282

Author: josevalim

lib/elixir/src/elixir_quote.erl

@@ -583,8 +583,10 @@ tail_list(Left, Right, Tail) when is_list(Right), is_list(Tail) ->
 
 tail_list(Left, Right, Tail) when is_list(Left) ->
   validate_list(Left),
-  [H | T] = lists:reverse(Tail ++ Left),
-  lists:reverse([{'|', [], [H, Right]} | T]).
+  case lists:reverse(Tail ++ Left) of
+    [H | T] -> lists:reverse([{'|', [], [H, Right]} | T]);
+    [] -> argument_error(<<"unquote_splicing/1 failed because it attempted to splice an empty list before tail position">>)
+  end.
 
 validate_list(List) ->
   case valid_ast_list(List) of

lib/elixir/test/elixir/kernel/quote_test.exs

@@ -200,10 +200,17 @@ defmodule Kernel.QuoteTest do
     contents = [1, 2, 3]
 
     assert quote(do: [unquote_splicing(contents) | [1, 2, 3]]) == [1, 2, 3, 1, 2, 3]
-
     assert quote(do: [unquote_splicing(contents) | val]) == quote(do: [1, 2, 3 | val])
-
     assert quote(do: [unquote_splicing(contents) | unquote([4])]) == quote(do: [1, 2, 3, 4])
+
+    # Empty splicing
+    assert quote(do: [unquote_splicing([]) | [1, 2, 3]]) == [1, 2, 3]
+    assert quote(do: [0, unquote_splicing([]) | [1, 2, 3]]) == [0, 1, 2, 3]
+    assert quote(do: [:head, unquote_splicing([]) | :tail]) == quote(do: [:head | :tail])
+
+    assert_raise ArgumentError,
+                 ~r"unquote_splicing/1 failed because it attempted to splice an empty list",
+                 fn -> quote(do: [unquote_splicing([]) | :tail]) end
   end
 
   test "splice on stab" do