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Comment by Joker_vD | Hacker News Reader

So... instead of mandating implementations to warn about (re)defining a reserved identifier, they introduce another class of "not yet reserved indentifiers" and advise implementations to warn about defining such identifiers in the user code — even though it's completely legal, — until the moment the implementation itself actually uses/defines such an identifier at which point warning about such redefinition in the user code — now illegal and UB — is no longer necessary or advised.

Am I completely misreading this or is this actually insane? Besides, there is already a huge swath of reserved identifiers in C, why do they feel the need to make an even larger chunk of names unavailable to the programmers?

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The problem is that the traditional wording of C meant that any variable named 'top' was technically UB, because it begins with `to'.

In practical terms, what compilers will do is, if C2y adds a 'togoodness' function, they will add a warning to C89-C2x modes saying "this is now a library function in C2y," or maybe even have an extension to use the new thing in earlier modes. This is what they already do in large part; it's semantic wording to make this behavior allowable without resorting to the full unlimited power of UB.

> Besides, there is already a huge swath of reserved identifiers in C, why do they feel the need to make an even larger chunk of names unavailable to the programmers?

The C23 change was mostly to downgrade some of the existing reserved identifiers from "reserved" to "potentially reserved". (It also added some new reserved and potentially reserved identifiers, but they seem reasonable to me.)

I still fail to see any practical difference between these two categories, except that the implementations are recommended to diagnose illegal-in-the-future uses of potentially reserved identifiers but are neither required nor recommended to diagnose actually illegal uses of reserved identifiers. There is also no way to distinguish p.r.i from r.i.

It also means that if an identifier becomes potentially reserved in C23 and reserved in C3X, then compiling a valid C11 program that uses it as C23 will give you a warning, which you can fix and then compile resulting valid C23 program as C3X without any problem; but compiling such a C11 program straight up as C3X will give you no warning and a program with UB.

Seriously, it boggles my mind. Just a) require diagnostics for invalid uses of reserved identifiers starting from C23, b) don't introduce new reserved identifiers, there is already a huge amount of them.

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