[Bug middle-end/104486] New: if constexpr versus -Wtype-limits

[Bug middle-end/104486] New: if constexpr versus -Wtype-limits

[drepper.fsp+rhbz at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104486

            Bug ID: 104486
           Summary: if constexpr versus -Wtype-limits
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp+rhbz at gmail dot com
  Target Milestone: ---

One use of 'if constexpr' is to handle different type sizes.  This is a simple
example:

#include <climits>

using some_type = int;

some_type f();

bool foo()
{
  if constexpr (sizeof(some_type) == sizeof(int))
    return f() == INT_MAX;
  else if constexpr (sizeof(some_type) == sizeof(long))
    return f() == LONG_MAX;
  else
    return f() == LONG_LONG_MAX;
}


Compiling this code with -Wtype-limits produces warnings:

g++ -std=gnu++20 -c u.cc -Wtype-limits
u.cc: In function ‘bool foo()’:
u.cc:12:16: warning: comparison is always false due to limited range of data
type [-Wtype-limits]
   12 |     return f() == LONG_MAX;
      |                ^
u.cc:14:16: warning: comparison is always false due to limited range of data
type [-Wtype-limits]
   14 |     return f() == LONG_LONG_MAX;
      |                ^

These statements are guarded by 'if constexpr' which causes the code in the
respective branch to not be active.  The warnings should be disabled for those
code blocks.  Again, the whole point of writing code like this is to avoid
overflow errors and the like.

[Bug middle-end/104486] if constexpr versus -Wtype-limits

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104486

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-02-10
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic

[Bug middle-end/104486] if constexpr versus -Wtype-limits

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104486

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Perhaps we should ignore the warning if in_discarded_stmt ?
Though it really matters where exactly the warning is reported, because
in_discarded_stmt isn't set when parsing a template with if constexpr in it
(after all, the condition might be dependent).
Other option would be to temporarily disable selected warnings around where we
set/restore in_discarded_stmt but also do it if processing_template_decl and
the condition isn't dependent and folds to false or true.