Type mismatch when inferring with union types?

[dwijnand]

minimized code

... Sorry, I don't have a minimised form. But the error is reproducible in #6744, and the error detail is:

[error] -- [E007] Type Mismatch Error: /d/dotty/compiler/src/dotty/tools/dotc/printing/Formatting.scala:215:46
[error] 215 |    val toExplain: List[(String, Recorded)] = seen.toList.flatMap {
[error]     |                                              ^
[error]     |Found:    List[(String, Object & dotty.tools.dotc.printing.Showable)]
[error]     |Required: List[(String, dotty.tools.dotc.printing.Formatting.Recorded)]
[error] 216 |      case (key, entry :: Nil) =>
[error] 217 |        if (needsExplanation(entry)) (key.str, entry) :: Nil else Nil
[error] 218 |      case (key, entries) =>
[error] 219 |        for (alt <- entries) yield {
[error] 220 |          val tickedString = seen.record(key.str, key.isType, alt)
[error] 221 |          (tickedString, alt)
[error] 222 |        }
[error] 223 |    }.sortBy(_._1)
[error] one error found

expectation

It shouldn't error, as proven by helping the inferencer out by assigning the result of the flatMap op to val res: List[(String, Recorded)] and then returning that.

[odersky]

It's a union type widening that ruins it. Nothing we can do about it right now. But good to know the reason it failed.