Given (T ty1) and ty2, we were computing the fingerprint of the
application (T ty1 ty2) by combining the two fingerprints from (T ty1)
and ty2.  But that gives a different answer to combinging the three
fingerprints from T, ty1, and ty2, which is what happens if you
build the type all at once.  Urk!

Fixes Trac #5962

MERGED from commit f35ebbd5