Why would I write a parser that almost-but-not-quite matches the compiler's own parser, when I could just use the compiler's parser directly? I don't want to write a parser, and I especially don't want to debug weird corner cases where my implementation diverges somehow. I just want to write some code that goes like, for each field in T, do X.

C++ metaprogramming is bad, but the problem there is the C++ part, not the metaprogramming-in-the-language part.