[namespace.std] Allow rejection of programs when they add declarations to the namespace std

[TymianekPL]

Changing the behaviour from undefined to ill-formed (no diagnostics required) in [namespace.std] when a program attempts to add a declaration to the namespace std.

[jensmaurer]

This change is not editorial.

Please file an LWG issue. (I do agree with the change, though.)

[TymianekPL]

The issue has been filed. I assume I should file another LWG issue to change "declarations or definitions" to "declarations" rather than make a PR?
I already made the commit in a cplusplus/draft fork, so just making sure ^^

[frederick-vs-ja]

I think there're many other occurrences of "undefined behavior" in the library wording intend to allow, but not require, implementations to reject compilation. So, "ill-formed, no diagnostic required" should be used instead.

LWG3142 is addressing one such case, although I think we should the make whole [res.on.functions] say IFNDR.

[TymianekPL]

I filed an LWG yesterday correcting namespace std, posix, future namespaces, reserved items in the standard library and header contents to say ill-formed, no diagnostics required ^^