[Bug c++/100956] New: Unused variable warnings ignore "if constexpr" blocks where variables are conditionally used

[Bug c++/100956] New: Unused variable warnings ignore "if constexpr" blocks where variables are conditionally used

[mattreecebentley at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100956

            Bug ID: 100956
           Summary: Unused variable warnings ignore "if constexpr" blocks
                    where variables are conditionally used
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mattreecebentley at gmail dot com
  Target Milestone: ---

For example, as part of a container class:

void remove_memory_blocks(pointer back_element_in_final_block)
{
   if constexpr(!std::is_trivially_destructible<element_type>::value)
   {
      // destroy each element in each memory block until the back_element is
reached
   }

   // remove memory blocks
}


If element_type is_trivially_destructible, G++ will warn that
back_element_in_final_block is unused every time the function is called.

Ideally this warning should be performed before constexpr blocks are removed by
a parser (I have no idea what the procedure is for GCC, I'm just guessing here)

[Bug c++/100956] Unused variable warnings ignore "if constexpr" blocks where variables are conditionally used

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100956

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The warning is already fixed in the GCC 10 branch.

N.B. if you have a loop that does nothing but run trivial destructors, GCC will
optimize it out anyway.

*** This bug has been marked as a duplicate of bug 81676 ***

[Bug c++/100956] Unused variable warnings ignore "if constexpr" blocks where variables are conditionally used

[mattreecebentley at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100956

--- Comment #2 from Matt Bentley <mattreecebentley at gmail dot com> ---
Thank you - I'm aware GCC might optimize it out (and failed to test with
GCC10), at least in O2 mode, but other compilers might not, hence the code.