From c6b0d6dce5709c61e549e850c2615fd288e27ba9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 15 Mar 2011 07:55:44 +0100
Subject: sys_core_fold: Eliminate incorrect warning

The compiler (sys_core_fold) tries to avoid constructing tuples
in case expressions. The following code:

c(A, B) ->
     case {A,B} of
         {ok,X} -> X;
         {_,_} -> error
     end.

will be rewritten so that no tuple is built. If a clause
requires a tuple to be built as in this code:

c(A, B) ->
     case {A,B} of
         {ok,X} -> X;
         V -> V    %The tuple will be built here
     end.

the tuple will be built in the clause(s) in which it is needed.

If the value returned from the case is not used as in this code:

c(A, B) ->
    case {A,B} of
	V -> V     %Warning: a term is constructed, but never used
    end,
    ok.

there will be an incorrect warning. Basically, what happens is
that the code is reduced to:

c(A, B) ->
    {A,B},         %Warning: a term is constructed, but never used
    ok.

and the optimizer sees that the {A,B} tuple can't possibly be used.

Eliminate the warning by adding a 'compiler_generated' annotation
to the tuple.

Reported-by: Kostis Sagonas
---
 lib/compiler/test/warnings_SUITE.erl | 10 ++++++++++
 1 file changed, 10 insertions(+)

(limited to 'lib/compiler/test/warnings_SUITE.erl')

diff --git a/lib/compiler/test/warnings_SUITE.erl b/lib/compiler/test/warnings_SUITE.erl
index 8cc3ca4199..dd18a6e1a3 100644
--- a/lib/compiler/test/warnings_SUITE.erl
+++ b/lib/compiler/test/warnings_SUITE.erl
@@ -453,6 +453,16 @@ effect(Config) when is_list(Config) ->
              	  true -> ok
                 end,
                 ok.
+
+             m8(A, B) ->
+                case {A,B} of
+                  V -> V
+                end,
+                ok.
+
+             m9(Bs) ->
+                [{B,ok} = {B,foo:bar(B)} || B <- Bs],
+                ok.
              ">>,
 	   [],
 	   {warnings,[{5,sys_core_fold,{no_effect,{erlang,is_integer,1}}},
-- 
cgit v1.2.3