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

["mattreecebentley at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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)