Correctly propagating a `decltype(auto)` variable from a function

That’s the simplest and most clear way to write it:

template <typename F>
auto invoke_log_return(F&& f)
{ 
    auto result = f();
    std::printf("    ...logging here... %s\n", result.foo());    
    return result;
}

The GCC gets the right (no needless copies or moves) expected result:

    s()

in main

prvalue
    s()
    ...logging here... Foo!

lvalue
    s(const s&)
    ...logging here... Foo!

xvalue
    s(s&&)
    ...logging here... Foo!

So if code is clear, have ever the same functionality but is’t optimized to run as much as the competitors does it’s a compiler optimization failure and clang should work it out. That’s the kind of problem that make lot more sense solved in the tool instead the application layer implementation.

https://gcc.godbolt.org/z/50u-hT