[Bug c++/88683] New: nan <= inf not accepted as constant expression

[mjansche at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88683

            Bug ID: 88683
           Summary: nan <= inf not accepted as constant expression
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mjansche at google dot com
  Target Milestone: ---

I am observing a regression between g++-7.4.0 and g++-8.2.0 when compiling the
following code:

#include <iostream>
#include <limits>
int main() {
  constexpr double nan = std::numeric_limits<double>::quiet_NaN();
  constexpr double inf = std::numeric_limits<double>::infinity();
  constexpr bool inf_ge_nan = inf >= nan;  // accepted
  std::cout << inf_ge_nan << std::endl;
  constexpr bool nan_le_inf = nan <= inf;  // rejected: not a constant
expression
  std::cout << nan_le_inf << std::endl;
  return 0;
}

I have read section 5.19 [expr.const] of the C++11 standard a few times and
don't see why the expression (nan <= inf) wouldn't be a constant expression in
the above code.

This is especially puzzling as (inf >= nan) is considered a constant expression
yet (nan <= inf) is not. At a minimum I would expect (inf >= nan) to be a
constant expression iff (nan <= inf) is.

I wouldn't make too much of the observation that this is a regression between
7.4.0 and 8.2.0: I've seen similar issues in g++-7.3.0, but haven't been able
to reproduce them in isolation.


Details:

$ /usr/local/stow/gcc-7.4.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/stow/gcc-7.4.0/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/stow/gcc-7.4.0/libexec/gcc/x86_64-pc-linux-gnu/7.4.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-7.4.0/configure --prefix=/usr/local/stow/gcc-7.4.0
Thread model: posix
gcc version 7.4.0 (GCC) 
$ /usr/local/stow/gcc-8.2.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/stow/gcc-8.2.0/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/stow/gcc-8.2.0/libexec/gcc/x86_64-pc-linux-gnu/8.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-8.2.0/configure --prefix=/usr/local/stow/gcc-8.2.0
Thread model: posix
gcc version 8.2.0 (GCC) 
$ /usr/local/stow/gcc-7.4.0/bin/g++ -std=c++11 -Wall -Wextra nan_le_inf.cc 
$ /usr/local/stow/gcc-8.2.0/bin/g++ -std=c++11 -Wall -Wextra nan_le_inf.cc 
nan_le_inf.cc: In function ‘int main()’:
nan_le_inf.cc:8:35: error: ‘(+QNaN <= +Inf)’ is not a constant expression
   constexpr bool nan_le_inf = nan <= inf;  // rejected: not a constant
expression
                               ~~~~^~~~~~