https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113064
Patrick Palka <ppalka at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |ppalka at gcc dot gnu.org --- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> --- (In reply to m.cencora from comment #4) > This also might be a just another symptom of the same root cause: > > struct bar > { > bar() = default; > > bar(const bar&); > bar(bar&&); > > bar& operator=(const bar&); > bar& operator=(bar&&); > }; > > struct foo > { > operator const bar& () const &; > > operator bar& () &; > > operator bar&&() &&; > }; > > void test() > { > bar a = foo{}; // ok It seems this copy-init is valid according to http://eel.is/c++draft/dcl.init#general-16.6.3 -- we first choose the best conversion function according to the destination type bar (and independent of bar's constructors) which in this case is operator bar&&() &&, and then perform overload resolution of bar's constructors using the result of that conversion function as the argument, which leads to bar(bar&&) unambiguously winning. > > a = foo{}; // not ok - ambiguous call, but why? &&-qualified looks like > a better match Whereas here, we perform the expected overload resolution of bar's constructor set using the original argument, which gives two viable candidates bar(const bar&) through operator const bar&() const& bar(bar&&) through operator bar&&() && and user-defined conversion sequences that use different conversion functions are incomparable, so the candidates are ambiguous. > > foo f; > a = f; // ok > > a = static_cast<const foo&>(foo{}); // ok > }