[Bug tree-optimization/110536] [12/13/14 regression] Bogus -Wstringop-overflow warning in std::transform

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110536

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2025-03-28
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Eric Niebler from comment #0)
> Compile the following with -O3 -std=c++17 -Wall
> 
> <<<<<<BEGIN
> 
> #include <algorithm>
> #include <cstdint>
> #include <vector>
> 
> template <typename TypeParam, typename T>
> std::vector<TypeParam>
> make_type_param_vector(std::initializer_list<T> const& init_list) {
>   // std::vector<T> input{init_list}; //uncomment to remove warning
>   std::vector<TypeParam> vec(init_list.size());

GCC is unable to tell that vec.size() == init_list.size() after this
constructor and so it's worried that vec.begin() might not point to anything.

Adding this makes the warnings go away:

  std::vector<TypeParam> vec(init_list.size());
  if (vec.size() != init_list.size())
    __builtin_unreachable();

>   std::transform(std::cbegin(init_list), std::cend(init_list),
> std::begin(vec), [](auto const& e) {


I tried adding if (size() != __l.size()) __builtin_unreachable() as the last
line of the vector(initializer_list<T>) constructor but it didn't help.