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/compiler/src/v3_core.erl | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

(limited to 'lib/compiler/src')

diff --git a/lib/compiler/src/v3_core.erl b/lib/compiler/src/v3_core.erl
index 0f80eb68fa..b96d3df8fe 100644
--- a/lib/compiler/src/v3_core.erl
+++ b/lib/compiler/src/v3_core.erl
@@ -1640,10 +1640,19 @@ get_unit({bin_element,_,_,_,Flags}) ->
     U.
 
 %% is_guard_test(Expression) -> true | false.
-%%  Test if a general expression is a guard test.  Use erl_lint here
-%%  as it now allows sys_pre_expand transformed source.
-
-is_guard_test(E) -> erl_lint:is_guard_test(E).
+%%  Test if a general expression is a guard test.
+%%
+%%  Note that a local function overrides a BIF with the same name.
+%%  For example, if there is a local function named is_list/1,
+%%  any unqualified call to is_list/1 will be to the local function.
+%%  The guard function must be explicitly called as erlang:is_list/1.
+
+is_guard_test(E) ->
+    %% erl_expand_records has added a module prefix to any call
+    %% to a BIF or imported function. Any call without a module
+    %% prefix that remains must therefore be to a local function.
+    IsOverridden = fun({_,_}) -> true end,
+    erl_lint:is_guard_test(E, [], IsOverridden).
 
 %% novars(Expr, State) -> {Novars,[PreExpr],State}.
 %%  Generate a novars expression, basically a call or a safe.  At this
-- 
cgit v1.2.3