Compiler panic with "proc" and GADTs in GHC 8.6.5
This is probably related to #9985 (closed), but that says that GADT patterns are prohibited in
proc so presumably I should have gotten an error message. Anyway, I got a compiler panic in GHC 8.6.5 and it asked me to report the bug. So here you are.
Minimal demonstration: Main.hs
Compiler output: out.txt