Never allow a comprehension to behave like a guard

Author: José Valim

lib/elixir/lib/kernel/special_forms.ex

@@ -813,6 +813,19 @@ defmodule Kernel.SpecialForms do
       iex> lc <<r::8, g::8, b::8>> inbits pixels, do: {r, g, b}
       [{213,45,132},{64,76,32},{76,0,0},{234,32,15}]
 
+  Note: Differently from Erlang, Elixir comprehensions filters
+  never behave as guards when it comes to errors. Errors in
+  list comprehensions will always be raised. Consider this
+  Erlang example:
+
+      erl> [I || I <- [1,2,3], hd(I)].
+      []
+
+  In Elixir, it will raise:
+
+      iex> lc i inlist [1,2,3], hd(i), do: i
+      ** (ArgumentError) argument error
+
   """
   defmacro lc(args)
 

lib/elixir/src/elixir_translator.erl

@@ -713,5 +713,7 @@ translate_comprehension_clause(_Meta, {inlist, Meta, [Left, Right]}, S) ->
   { { generate, ?line(Meta), TLeft, TRight }, SL };
 
 translate_comprehension_clause(Meta, X, S) ->
+  Line = ?line(Meta),
   { TX, TS } = translate_each(X, S),
-  elixir_tree_helpers:convert_to_boolean(?line(Meta), TX, true, false, TS).
+  { BX, BS } = elixir_tree_helpers:convert_to_boolean(Line, TX, true, false, TS),
+  { { match, Line, { var, Line, '_' }, BX }, BS }.