[Bug c++/107531] New: List of references calls destructors, add warning?

[nightstrike at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107531

            Bug ID: 107531
           Summary: List of references calls destructors, add warning?
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nightstrike at gmail dot com
  Target Milestone: ---

The following code results in a destructor being calling twice:

#include <initializer_list>
struct S {
    S() { __builtin_printf("ctor\n"); }
    ~S() { __builtin_printf("dtor\n"); }
};

template<typename ... Args>
void f(Args const & ... args) {
    for (auto const & s: { args... })
        ;
}

int main() {
    S s;
    f(s);
}

$ ./a.out
ctor
dtor
dtor


Changing line 9 to use pointers fixes it:

for (auto const * s: { &args... })

$ ./a.out
ctor
dtor


I think that if my code is exploiting some kind of bad behavior (and it likely
is), a warning at -Wall would be a nice addition (currently, there is none). 
If what I'm doing is allowed, I don't think calling a destructor twice is the
best idea :) :).  I'd be betting money on the first thing, though.  Maybe
adding something from -fsanitize=undefined would be an option?