Someone may prove me wrong here, but to me it looks like this would only prevent ASSERT from being used as the name of a PL/PgSQL variable.
The comment at the beginning of pl_scanner.c seems to suggest same.
yes, there are no too much possibilities, how to do it.
Effective implementation needs a special PLpgSQL statement - but I am 100% happy with proposed syntax. We introduce a new special keyword just for one simple construct.
A some languages has a generic PRAGMA keyword. So I would be much more happy with something like
PRAGMA Assert(found);
It is much more close to ADA, and it allows a reuse of new keyword for any other usage in future (your proposal is too simply, without possibility open new doors in future). And we can define a some standard predefined asserts too - like Assert, AssertNotNull, AssertNotEmpty, ...
other issue - A asserts macros has one or two parameters usually. You should to support two params assert (with message).
