[Bug c++/19199] [3.3/3.4/4.0 Regression] Wrong warning about returning a reference to a temporary

["gdr at integrable-solutions dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From gdr at integrable-solutions dot net  2005-01-02 00:43 -------
Subject: Re:  [3.3/3.4/4.0 Regression] Wrong warning about returning a reference to a temporary

"pinskia at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| (In reply to comment #2)
| > Why would this cause wrong code?  It's just a cast from an enum to an int, 
| > I don't see what's wrong with that except that it's ugly. 
| >  
| > You have to actually show that this causes wrong code, not just make your 
| > usual exaggerated claims ;-) 
| 
| Ok, the reason why it is wrong code is because we return an address to a temporary which we can 
| overwrite.  Yes this is wrong code.  The diagnostic is just an indication that it will be wrong code.

"rth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| When references are involved, like this, reduction to MAX_EXPR is wrong to 
| begin with.  The cast is only an additional symptom.

Both Andrew and RTH are obviously right.  This is wrong-code
generation. As a "proof", I've slighlty modified the testcase to
include an assertion on the address of the object that realizes the
min.  It fails, as can be expected from the warning.
Here we go.

% cat 19199.C && g++ 19199.C -o 19199 && ./19199    
#include <assert.h>

enum Foo { A, B };

template<typename T> const T &qMin(const T &a, const T &b)
{ return a < b ? a : b; }

int main(int,  char **)
{
   Foo f = A;
   Foo g = B;
   const Foo& h = qMin(f, g);
   assert(&h == &f || &h == &g);
   return 0;
}
19199.C: In function 'const T& qMin(const T&, const T&) [with T = Foo]':
19199.C:12:   instantiated from here
19199.C:6: warning: returning reference to temporary
19199: 19199.C:13: int main(int, char**): Assertion `&h == &f || &h == &g' failed.
zsh: 10279 abort      ./19199


-- Gaby



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19199