[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 #19 from Dan Urosu <durosu at yahoo dot com> ---
(In reply to Marek Polacek from comment #18)
> But is unwrap_1 called in such a way that 'r' binds to a temporary?

Did you mean 'u' binds to a temporary?

In this case r1 is a temporary object and a reference to its inner value is
returned by the function.
The warning might have not been intended for this case, but all I'm saying is
that this example shows a very common case of a dangling reference. (and is not
flagged)

full 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) {
  Wrapper<T> r1(r); // << TEMPORARY 
  return r1.value(); // << TEMPORARY 
}

#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_2(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); // <<<<< here
#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;
}