errors when extending records in invalid ways

[endlessoblivion]

In Elm 0.18:

type T = T {}
type alias A a = { a | x : () }

f : A T -> ()
f { x } = x

still results in:

elm-make: Used toSrcType on a type that is not well-formed
elm-make: thread blocked indefinitely in an MVar operation

despite this commit for 0.16 claiming a fix.

Related: #917

Note from Evan: Issue #1660 points out that you can have problems when it is a record, but has duplicate fields. It seems like kind checking would have to have a bit of extra cleverness to catch that situation as well.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Tracking in #1684

Thank you for the report!