How to cause a compiler warning?

[Richard Stallman]

[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]

I would like to make the cond* macro generate warnings in certain
cases, when it is being copiled.  What is the recommended way to do
that?

When I implemented constrained variables, nested constrained variables
simply worked, with no extra code.  But since some people said they were
confusing, I am thinking of issuing a warning for those cases.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)