[Bug c++/56292] New: False positive for constexpr arithmetics (-Wconversion)

[Bug c++/56292] New: False positive for constexpr arithmetics (-Wconversion)

[lcid-fire at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

             Bug #: 56292
           Summary: False positive for constexpr arithmetics
                    (-Wconversion)
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: lcid-fire@gmx.net


For the following code one gets 'conversion' diagnostics, while the compiler
should be able to compute, that it is actually fine.

#include <cstdint>
constexpr std::uint8_t func() { return 2; }
std::uint8_t value = func() + 2;

Results in:
warning: conversion to ‘uint8_t {aka unsigned char}’ from ‘int’ may alter its
value [-Wconversion]

Diagnostics works fine if you replace 'func()' with a number.

[Bug c++/56292] False positive for constexpr arithmetics (-Wconversion)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-12 09:03:44 UTC ---
func() + 2 is evaluated as (unsigned char) (((int) func ()) + 2), so when the
-Wconversion warning is diagnosed before evaluating all the constexpr
expressions into constants if possible, the warning is reasonable.
Consider
constexpr std::uint8_t func() { return 0xff; }
std::uint8_t value = func() + 2;

[Bug c++/56292] False positive for constexpr arithmetics (-Wconversion)

[lcid-fire at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

--- Comment #2 from lcid-fire at gmx dot net 2013-02-12 10:13:17 UTC ---
But should it be evaluated before constexpr are processed?

[Bug c++/56292] False positive for constexpr arithmetics (-Wconversion)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> 2013-02-12 12:29:28 UTC ---
Does it also warn if you make value a constexpr?

[Bug c++/56292] False positive for constexpr arithmetics (-Wconversion)

[lcid-fire at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

--- Comment #4 from lcid-fire at gmx dot net 2013-02-12 15:23:58 UTC ---
constexpr std::uint8_t value = func() + 2;

does generate the same warning.

[Bug c++/56292] False positive for constexpr arithmetics (-Wconversion)

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56292

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.9.0, 4.9.1, 5.0
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This is fixed in 4.9.0.