Well at first almost everyone seemed to have thought it had to be complex to write an Ada compiler. It wasn't until the Robert Dewar talked (for some time) with Richard Stallman that he saw à way to drastically simplify the implementation of an Ada compiler.
This story and hommage by dwheeler https://www.dwheeler.com/essays/make-it-simple-dewar.html was one of the nicest things written on the late Robert Dewar... and on RMS !
The GNAT Front End is a large and complex piece of software and the language is very powerful thanks to all the features packed in the compiler.
On the other hand, for compliance with the standard there is a complete test suite : ACATS... It should help...