https://en.wikipedia.org/wiki/Edison_Design_Group

TLDR EDG C++ is a compiler frontend developed by EDG since the 80s which has been used under the hood for a whole bunch of commercial C and C++ compilers, static analyzers, linters, and IDE code completion tools.

Odds are if you are familiar with a closed source tool in one of those categories above, there's a decent chance it uses EDG's frontend somewhere in it under the hood.