[Bug c++/115987] False "possibly dangling reference" false positive when having an extra template parameter

[durosu at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115987

--- Comment #8 from Dan Urosu <durosu at yahoo dot com> ---
Why does not warn for unwarp_1?
The mere addition of an unused template parameter triggers this warning.

Also "decorating the unwrap_2 function to tell the compiler not to warn there."
does not work if we have a macro calling unwrap_2. (Which we would use in the
real code)

#include <iostream>
#include <optional>

// Compile: gcc -std=c++17 -Wall -Wextra -Werror -lstdc++ main_unwrap.cpp -o
main_unwrap
/**
'unwrap_1' and 'unwrap_2' are functionally identical.
By introducing an additional template parameter, the code fails to compile,
when unwrap_2 is used.
The code should not give errors because the lifetime of the returned reference
is tied to the lifetime of the 'wrapper' object.
*/

template <typename T>
struct Wrapper {
    const T& value() const { return val.value(); }
    T& value() { return val.value(); }
    std::optional<T> val;
};


template <typename T>
    const T& unwrap_1(const Wrapper<T>& r) {
  return r.value();
}

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
template <typename T, typename FUNC>
    const T& unwrap_2(const Wrapper<T>& r, FUNC&&) { // note FUNC is not even
used
  return r.value();
}
#define UNWRAP_2(x, y) unwrap(x, y)
#pragma GCC diagnostic pop


#define WITH_FUNC_PARAM

int main(int, char**) {
  const Wrapper<int> w{1234};
#ifndef WITH_FUNC_PARAM
  const auto& u = unwrap_1(w);
#else
  const auto& u = UNWRAP_2(w, 1); // This fails with error: possibly dangling
reference to a temporary [-Werror=dangling-reference]
#endif
  printf("Unwrapped value : %d\n", u);

  return 0;
}