https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113527
Bug ID: 113527 Summary: Missed optimization [[assume]] attribute Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: eric-bugs at omnifarious dot org Target Milestone: --- This code https://godbolt.org/z/jeYGcr5sv still generates all the exception handling code. It treats the assume attribute more like 'likely' than 'assume'. It's supposed to be undefined behavior for the assumption to be false. Here is the code replicated here: #include <variant> struct Potato { explicit Potato(int); Potato(const Potato & other) noexcept; }; using V = std::variant<Potato, int, double>; template <class... Ts_> struct overloaded : Ts_... { using Ts_::operator()...; }; auto f(const V & v) -> int { [[assume(! v.valueless_by_exception())]]; return visit(overloaded{ [](int) { return 144; }, [](double) { return 27; }, [](const Potato &) { return 50; } }, v); } You can make the exception handling code go away by simply using "if (v.valueless_by_exception())" and returning some random value in the false case. But, if the spirit of the keyword is to be observed, you should be fine just doing a straight up table lookup and using the result without even checking the type tag at all.