I think I figured out the “not unranked” meta issue!!! https://github.com/elm-lang/elm-compiler/issues/1613 CONTEXT: When types fail to unify, they become Error. Error means we can avoid displaying cascading errors and only show the actually broken thing. BEFORE: In unification, we just quit when we saw an Error and some other type. No cascade! No obvious problem! AFTER: We actually need to propagate the Error in unification. If a type must unify with an Error, we CANNOT be confident that this will succeed once the Error is fixed. So it needs to be an Error too to avoid all bad cascades. But more importantly for this bug, the `merge` function produces a variable with (min rank1 rank2) as its rank! I think the root of the bug was skipping that `min` call when there are very particular cascading errors. Fixes https://github.com/elm-lang/elm-compiler/issues/1525 Fixes https://github.com/elm-lang/elm-compiler/issues/1600 Fixes https://github.com/elm-lang/elm-compiler/issues/1609 Fixes https://github.com/elm-lang/elm-compiler/issues/1582 Fixes https://github.com/elm-lang/elm-compiler/issues/1638 Fixes https://github.com/elm-lang/elm-compiler/issues/1569 Fixes https://github.com/elm-lang/elm-compiler/issues/1657 Fixes https://github.com/elm-lang/elm-compiler/issues/1607 Fixes https://github.com/elm-lang/elm-compiler/issues/1633 Fixes https://github.com/elm-lang/error-message-catalog/issues/185