[Bug c++/47570] New: "one() >= 0" isn't constexpr for unsigned int, yet == and > is.

[yacwroy at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47570

           Summary: "one() >= 0" isn't constexpr for unsigned int, yet ==
                    and > is.
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: yacw...@gmail.com
            Target: x86_64-unknown-linux-gnu


Created attachment 23194
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23194
cxp_geq_uint.cpp

I reduced the code down as far as I could. It's fairly self-explanatory.

cxp_geq_uint.cpp
============================
unsigned int constexpr one()
  { return 1; }

int constexpr one_B()
  { return 1; }

int main()
  {
    // FAIL TO COMPILE:
    //static bool constexpr SC_huh = ((unsigned int)one()) >= ((unsigned
int)0);
    //static bool constexpr SC_huh = one() >= ((unsigned int)0);
    static bool constexpr SC_huh = one() >= 0;

    // COMPILE OK:
    //static bool constexpr SC_huh = ((one() == 0) || (one() > 0));
    //static bool constexpr SC_huh = one() == 0;
    //static bool constexpr SC_huh = one() > 0;
    //static bool constexpr SC_huh = one_B() >= 0;
    //static bool constexpr SC_huh = one() >= 1;

    return SC_huh;
  }
==============================
g++ cxp_geq_uint.cpp --std=c++0x
==============================
cxp_geq_uint.cpp: In function ‘int main()’:
cxp_geq_uint.cpp:12:43: error: ‘(1u, true)’ is not a constant expression
==============================

I can see no reason why >= shouldn't be constexpr in this case.

IIRC standard operators and literal casts always have defined behaviour for
standard unsigned integers.

As shown in the code, the problem goes away if:
- The value to the right of >= is greater than 0.
- The type is changed to (regular) int.
- The operation is changed to == or > (or combining these manually to form >=)


SPECS:
gcc: version 4.6.0 2010-12-30 (experimental) (svn = 168358)
 - manually patched by
(http://gcc.gnu.org/ml/gcc-patches/2007-04/msg00620.html)
 - patch shouldn't have any effect here.
ubuntu: 10.10 (64 bit)
intel: core2 duo


I searched the buglist for constexpr & unsigned, didn't see any dupes myself.


I guess this one probably won't be too hard to debug.
I might even have a go if I get some spare time.

HTH.
Simon.