Yes, i've heard this feedback before. I'm very much open to alternatives. [[requires no_unique_address]] is one i toyed with, but it is rather verbose. A non-alphanumeric syntax also has the benefit of not being even close to valid with an attribute-specifier in C++20.

On 4 May 2023, at 11:29, Bo Persson via Std-Proposals <> wrote:
On 2023-05-04 at 09:47, Lauri Vasama via Std-Proposals wrote:
Floating an idea i've been toying around with for a while concerning a
new syntax for attributes with observable effects on program behaviour. <>

Don't see that an exclamation mark is a good choice for "required", as
it elsewhere means "not".

Like [[packed]] is optional, but [[!packed]] should be required packing.
To me it look more like unpacked.