Since C++11 elision of copies of named local lvalues being returned is mandatory. In practice this was elided by older smart compilers and new compilers elide a whole more when returning.
That seems entirely unrelated to tail call optimisation though? For once, "return foo(bar, baz)" isn't returning a named value, it's returning an rvalue. Secondly, return value optimisation doesn't optimise away the stack frame. Thirdly, the standard isn't as strict as you make it sound: compilers may elide the copy if they can, but if they cannot they have to move, never copy.
