My understanding is that it is now implementation defined if $ is in the
identifier set, after having been removed entirely.
See https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3145.pdf

On Wed, Jul 5, 2023, 12:28 Hubert Tong via SG16 <sg16_at_[hidden]>
wrote:

> We adopted, in Varna, a paper that adds $ to the basic character set. The
> week after, the C committee decided (for the C23 DIS) that $ can be used
> within identifiers (including in the initial position).
>
> We should consider keeping C and C++ in sync.
>
> Note that we already have examples of u"text", etc. where identifier
> non-digits form the initial portion of non-identifier pp-tokens.
>
> -- HT
>
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