[Issue 21591] Mangling problem wrt. backrefs and function types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21591

--- Comment #1 from kinke <kinke at gmx.net> ---
Looking at dmd/dmangle.d, some missing type merge must be causing this (the
backrefs depend on equivalent types sharing the same `Type` instance).

--