Hi,

I would like to propose an attribute [[experimental(msg)]], with similar
semantics to [[deprecated(msg)]]. Even on (or specially on) stable
libraries the developers may wish to introduce a new type or function to
their libraries, but are uncertain if this is the final form the addition
would take.

Some users of such libraries may want to receive a diagnostic when using
experimental features. Other (more adventurous types) may be Okay with
adopting not-quite-fully-baked features. Abusing [[deprecated(msg)]] for
this purpose almost works, but has some clear disadvantages:

- It is obviously the wrong annotation, a casual reader may draw the wrong
conclusion.
- The compiler diagnostics typically include the word "deprecated" which is
confusing.
- Users of the library may wish to receive diagnostics about deprecated
features, but may be Okay with using new experimental features.

Having a new attribute would obviate these problems and give
library developers a path to introduce new functionality, with sufficient
diagnostics for folks that prefer to avoid them.


Regards,

-- 
Carlos O'Ryan
// -fno-exceptions delenda est