Compiler hangs on infinite type in badly written foldl lambda

[drewwestphal]

Referencing #1240 here as many similar (largely closed) issues reference it. Noted that @evancz suggested creating new issues in most cases, so I am doing so rather than piling on there--hope that is desired.

This causes the compiler to hang on 0.18.0 on a mac. (Also hangs elm reactor.)

listUnique lst =
    List.foldr
        (\elem uniq ->
            if List.member elem uniq then
                uniq
            else
                uniq :: elem
        )
        []
        lst

By breaking out the lambda into its own function (suggested on the elm slack), the compiler detects and errors on an infinite type (see here for that example)

[rtfeldman]

Quick note that this is still reproducible in 0.19, on the version of the binary that fixes #1743 (I cannot reproduce that issue, but I can still reproduce this one).