[Bug c++/55986] New: RHS of logical operators may render LHS unevaluated in constant-expression

[Bug c++/55986] New: RHS of logical operators may render LHS unevaluated in constant-expression

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

             Bug #: 55986
           Summary: RHS of logical operators may render LHS unevaluated in
                    constant-expression
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: potswa@mac.com


This compiles without incident, despite using a non-constant object in a
constant-expression:

int i;
static_assert( i || true, "" );
static_assert( ! ( i && false ), "" );

It appears to be assuming that || and && are commutative, but short-circuiting
only works in one direction.

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

--- Comment #1 from David Krauss <potswa at mac dot com> 2013-01-15 04:47:53 UTC ---
Passing `-O2` additionally causes it to skip a condition like `(i == 0 || i !=
0)`, so it looks like a general issue with expression simplification bypassing
ODR-use.

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

--- Comment #2 from David Krauss <potswa at mac dot com> 2013-01-15 04:48:47 UTC ---
Passing `-O2` additionally causes it to skip a condition like `(i == 0 || i !=
0)`, so it looks like a general issue with expression simplification bypassing
use.

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

--- Comment #3 from David Krauss <potswa at mac dot com> 2013-01-15 08:02:55 UTC ---
Likewise this is allowed:

switch( 0 )
case i-i: ;

I think this is a purposeful language extension, which could use a switch to
disable. It would be nice if static_assert were always strict.

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[yaghmour.shafik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

Shafik Yaghmour <yaghmour.shafik at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yaghmour.shafik at gmail dot com

--- Comment #4 from Shafik Yaghmour <yaghmour.shafik at gmail dot com> ---
This issue showed up recently in this Stackoverflow question:
http://stackoverflow.com/q/31526278/1708801

As far as I can tell there should at least be a diagnostic when using -pedantic

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[yaghmour.shafik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

--- Comment #5 from Shafik Yaghmour <yaghmour.shafik at gmail dot com> ---
It looks like this case from this Stackoverflow question
http://stackoverflow.com/q/32920229/1708801 is possibly related:

int main(int argc, char**)
{
    constexpr int a = argc * 0;
}

gcc treats `argc * 0` as a constant expression a comment indicates this is due
to constant folding and it seems possible that gcc is constant folding `i ||
true` as well.

[Bug c++/55986] RHS of logical operators may render LHS unevaluated in constant-expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55986

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
           Keywords|                            |accepts-invalid
   Target Milestone|---                         |6.0
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 6 with r6-4886 (note this is expected because we delay
simplifications until after semantics anlysis now).