https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91456
Bug ID: 91456
Summary: std::function and std::is_invocable_r do not
understand guaranteed elision
Product: gcc
Version: 9.1.1
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
// { dg-options "-std=gnu++17" }
// { dg-do compile { c++17 } }
#include <functional>
struct Immovable {
Immovable() = default;
Immovable(const Immovable&) = delete;
Immovable& operator=(const Immovable&) = delete;
};
Immovable get() { return {}; }
const Immovable i = get(); // OK
std::function<const Immovable()> f{&get}; // fails
const Immovable i2 = f();
It fails because std::function uses is_same<R1,R2> || is_convertible<R1,R2> to
check if the target function's return type can be converted to the
std::function return type. Immovable and const Immovable are not the same type,
and is_convertible checks whether construction or R2 from R1&& is well-formed,
so that fails too.
It should succeed in C++17, because guaranteed elision means there's no copy
from an rvalue.
