I'm referring to both syntax based (AST) macros, and text based (preprocessor) macros. The latter, of course, are much worse.
An example of the former is so-called "expression templates" in C++. I've seen them used to create a regular expression language using C++ expression templates. The author was quite proud of them, and indeed they were very clever.
However nice the execution, the concept was terrible. There was no way to visually tell that some ordinary code was actually doing regular expressions.
C++ expression templates had their day in the sun, but fortunately they seem to have been thrown onto the trash pile of sounds-like-a-good-idea-but-oops.
(I wrote an article showing how to do expression templates in D, mainly to answer criticisms that D couldn't do it, not because it was a good idea.)
