> This is standardizing vendor-specific attributes

Except the standard’s likely and unlikely attributes invented new syntax that is not drop-in compatible with clang’s and gcc’s attributes.

Where clang and gcc would use:

  if (__builtin_expect(x > 0, 1)) { … }

the standard uses:

  if (x > 0) [[likely]] { … }

It's definitely an unconventional syntax. In addition to the above, OpenMP and shader languages annotate the branch statement for parallelism or branch/predication hints. I can't think of precedent for C++ putting the hints in the branch targets. It does have some advantages, but it's not very intuitive. The first time I tried to use the new hints I did [[likely]] if(), which of course did nothing.

Doesn't look like it. The attribute goes on branches. This goes in the middle of basic blocks. Doesn't seem better to me, might try to find the rationale for the invention instead of standardising existing practice. That proposal shows the existing attributes with different syntax.