I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Thanks for making the STL go faster! Must go faster! 🏃 🚗 🦖

It is worth noting that @xiangfan-ms noted since these functions are not instantiated in some cases, it is possible that static_assertions in the body of these functions would fail to fire so these assertions need to be hoisted as constrains to calling the function.

@cdacamar that implies a bug in one of the std::forward overloads which needs a static_assert to enforce one of the Standard's requirements. I'm unable to reproduce it, though. This program:

#include <utility>
int main() {
    (void) std::forward<int&>(42);
}

triggers the static_assert even with the latest prod/fe compiler. Do I misunderstand the optimization, or ...?

Did you also compile with /permissive-? When I compile with /permissive- I get:

t.cpp(4): error C2101: '&' on constant

Which is correct if the function were replaced with a cast, but also does not instantiate the function so the static assertion is missing.

Did you also compile with /permissive-?

Aha! No, I did not. Do I surmise correctly that the optimization is only enabled in strict mode?

Yes, the optimization is only available when /permissive- is used or implied because two-phase is required for function template definition checking whether the attribute is applicable to the function.