[Bug c++/53792] [C++11][constexpr] improving compiler-time constexpr evaluation

[vincenzo.innocente at cern dot ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53792

--- Comment #2 from vincenzo Innocente <vincenzo.innocente at cern dot ch> 
2012-06-28 11:08:38 UTC ---
On 28 Jun, 2012, at 11:34 AM, rguenth at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53792
> 
> Richard Guenther <rguenth at gcc dot gnu.org> changed:
> 
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>             Status|UNCONFIRMED                 |NEW
>   Last reconfirmed|                            |2012-06-28
>     Ever Confirmed|0                           |1
> 
> --- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-28 
> 09:34:20 UTC ---
> Does the C++ standard require getIndex to be evaluated to a constant in getV?
No, a constexpr function can be used as a "normal" function. For instance if
getV was not inlined 
the correct behaviour is what we observe 
> Or is 'constexpr' only telling you that it is required ina context where
> evaluation to a constant is required (and thus arguments to a constexpr
> function are constrained)?
I would say yes (a constant needs to be evaluated using a constexpr function.
see page 148-150 (166-168 of my pdf) of the standard (7.1.5 point 3 and
following)
> 
My point is that the compiler does not identify that "getIndex" is a constant
expression in the context of foo
 nor in context of the following foo2 that indeed produce (as it should)
identical code to foo

int foo2() {
  const double a = v[getIndex("a",theMap)];
  const double b = v[getIndex("b",theMap)];

  if (a==b) return 1;
  return 0;

}