From 0baa07cdf2754748bbc2d969bf83f08c0976fb78 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Mon, 15 Aug 2016 15:34:06 +0200
Subject: Fix overridden BIFs

The filters in a list comprehension can be guard expressions or
an ordinary expressions.

If a guard expression is used as a filter, an exception will basically
mean the same as 'false':

  t() ->
    L = [{some_tag,42},an_atom],
    [X || X <- L, element(1, X) =:= some_tag]
    %% Returns [{some_tag,42}]

On the other hand, if an ordinary expression is used as a filter, there
will be an exception:

  my_element(N, T) -> element(N, T).

  t() ->
    L = [{some_tag,42},an_atom],
    [X || X <- L, my_element(1, X) =:= some_tag]
    %% Causes a 'badarg' exception when element(1, an_atom) is evaluated

It has been allowed for several releases to override a BIF with
a local function. Thus, if we define a function called element/2,
it will be called instead of the BIF element/2 within the module.
We must use the "erlang:" prefix to call the BIF.

Therefore, the following code is expected to work the same way as in
our second example above:

-compile({no_auto_import,[element/2]}).

element(N, T) ->
    erlang:element(N, T).

t() ->
    L = [{some_tag,42},an_atom],
    [X || X <- L, element(1, X) =:= some_tag].
    %% Causes a 'badarg' exception when element(1, an_atom) is evaluated

But the compiler refuses to compile the code with the following
diagnostic:

  call to local/imported function element/2 is illegal in guard
---
 lib/stdlib/test/qlc_SUITE.erl | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

(limited to 'lib/stdlib/test')

diff --git a/lib/stdlib/test/qlc_SUITE.erl b/lib/stdlib/test/qlc_SUITE.erl
index 2bd940020c..8c7d5a5fcf 100644
--- a/lib/stdlib/test/qlc_SUITE.erl
+++ b/lib/stdlib/test/qlc_SUITE.erl
@@ -58,7 +58,7 @@
 -export([ 
 	  badarg/1, nested_qlc/1, unused_var/1, lc/1, fun_clauses/1,
 	  filter_var/1, single/1, exported_var/1, generator_vars/1,
-	  nomatch/1, errors/1, pattern/1, 
+	  nomatch/1, errors/1, pattern/1, overridden_bif/1,
 
 	  eval/1, cursor/1, fold/1, eval_unique/1, eval_cache/1, append/1, 
 	  evaluator/1, string_to_handle/1, table/1, process_dies/1, 
@@ -126,7 +126,7 @@ groups() ->
     [{parse_transform, [],
       [badarg, nested_qlc, unused_var, lc, fun_clauses,
        filter_var, single, exported_var, generator_vars,
-       nomatch, errors, pattern]},
+       nomatch, errors, pattern, overridden_bif]},
      {evaluation, [],
       [eval, cursor, fold, eval_unique, eval_cache, append,
        evaluator, string_to_handle, table, process_dies, sort,
@@ -468,6 +468,23 @@ pattern(Config) when is_list(Config) ->
                          -record(k, {t,v}).\n">>, Ts),
     ok.
 
+%% Override a guard BIF with an imported or local function.
+overridden_bif(Config) ->
+    Ts = [
+	  <<"[2] = qlc:e(qlc:q([P || P <- [1,2,3], port(P)])),
+             [10] = qlc:e(qlc:q([P || P <- [0,9,10,11,12],
+                                      (is_reference(P) andalso P > 5)])),
+             Empty = gb_sets:empty(), Single = gb_sets:singleton(42),
+             GbSets = [Empty,Single],
+             [Single] = qlc:e(qlc:q([S || S <- GbSets, size(S) =/= 0]))
+            ">>
+	 ],
+    run(Config, "-import(gb_sets, [size/1]).
+                 -compile({no_auto_import, [size/1, is_reference/1]}).
+                 port(N) -> N rem 2 =:= 0.
+                 is_reference(N) -> N rem 10 =:= 0.\n", Ts),
+    ok.
+
 
 %% eval/2
 eval(Config) when is_list(Config) ->
-- 
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