Richard Eisenberg authored Sep 25, 2019 and Marge Bot committed Oct 03, 2019

This patch also changes the way we handle empty lists, simplifying
them somewhat. See Note [Empty lists]. Previously, we had to
special-case empty lists in the type-checker. Now no more!

Finally, this patch improves some documentation around the ir_inst
field used in the type-checker.

This breaks a test case, but I really think the problem is #17251,
not really related to this patch.

Test case: typecheck/should_compile/T13680