"restrict" in brackets - what is this?

[Ian Pilcher via Gcc-help <gcc-help@xxxxxxxxxxx>]

I am looking at the man page for inet_ntop(3) on my Fedora 38 system,
and it shows the following:

 const char *inet_ntop(int af, const void *restrict src,
                       char dst[restrict .size], socklen_t size);
                                ^^^^^^^^^^^^^^

I have never seen the underlined syntax used before.  Is this a GCC
extension, something from one of the newer C standards, or something
else?

Sorry for the potentially stupid question, but none of my searches have
turned up anything.

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