Why check for -fdefer-type-errors in metaTyVarUpdateOK?

Function checkTypeEq changes its behavior depending on the presence of -fdefer-type-errors in an obscure case around heterogeneous equalities; see the code in go_co. This is undocumented (in the code), and neither Simon nor I can figure out why it's done.

Task: figure this out, and either document or remove this behavior.

Edited Dec 22, 2020 by Richard Eisenberg

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information