[tomsmeding]

Yes, the -std flag would definitely still exist: the thing being changed by that flag is a lot of stuff in the parser, some stuff in the standard library, and some semantics bits further towards the backend, but at some point the versions converge and go through the same optimisation pipeline. A pipeline that definitely has seen improvements in more recent versions, so it's totally worth it to use a new compiler with an old language standard on old code: it might very well produce a faster executable. Just using an older compiler would also revert back to the old performance.

Additionally, a C/C++ compiler in an older standard mode would disable certain features, of course, but not completely: it's still able to give diagnistics in the vein of "this thing isn't valid with the current version setting, but it would be valid in C++17", for example. That's useful to a programmer.

[marvy]

Also useful in the other direction: your code works now, but in C++17 that's a keyword so you'll have to rename it.