[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 
#include 
int main() {
  constexpr double nan = std::numeric_limits::quiet_NaN();
  constexpr double inf = std::numeric_limits::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
   ^~

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

[mjansche at google dot com]

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

--- Comment #2 from Martin Jansche  ---
Yeah, looks definitely related. Details may depend on the ultimate diagnosis. 

Similar observation here that it matters whether NaN appears on the left-hand
side vs. right-hand side of a comparison (cf.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88173#c5).

Additional notes:

* When/whether this surfaces depends to some extent on the version of gcc. I
first thought it was a simple regression between gcc-7 and gcc-8, but it's more
complex than that.

* The title of Bug 88173 could be interpreted to leave open the possibility
that this might be a library issue, i.e. std::numeric_limits<>::quiet_NaN() not
being declared constexpr. But it's definitely not a library issue.

Anyway, I agree with the recommended disposition as DUP. But I'm new around
here and don't know who gets to close this as DUP. Happy to do it myself, just
don't want to jump the gun.