Map type inference not working when map creation expression is not an argument

[erszcz]

As a followup to #429 I've stumbled upon test/should_fail/covariant_map_keys_fail.erl. There's no error reported for kaboom/0 by default (even with the --infer flag), but there is if we apply the following change:

$ git diff
diff --git a/test/should_fail/covariant_map_keys_fail.erl b/test/should_fail/covariant_map_keys_fail.erl
index 99c5883..98e38f0 100644
--- a/test/should_fail/covariant_map_keys_fail.erl
+++ b/test/should_fail/covariant_map_keys_fail.erl
@@ -10,4 +10,6 @@ good(#{ good := X }) -> X.
 not_good(M) -> good(M). %% This call should fail

 -spec kaboom() -> integer().
-kaboom() -> not_good(#{ bad => 0 }).
+kaboom() ->
+    M = #{ bad => 0 },
+    not_good(M).

This shows that map type inference does not always work as expected. It might or might not be related to #378. I have a hunch it's related, as it's inference of a nested map creation expression which doesn't work as expected, not a top-level one.