Improve error: the pattern contains an unaccounted type parameter

[TomasMikula]

Compiler version

3.6.4

Minimized code

sealed trait KV[K, V]
sealed trait Append[Init, KV]

type KeysOf[KVs] = KVs match
  case KV[k, v] => k
  case Append[init, KV[k, v]] => KeysOf[init] | k

// OK
summon[KeysOf[KV["x", Int]] =:= "x"]

// Error
summon[KeysOf[Append[KV["x", Int], KV["y", String]]] =:= ("x" | "y")]

Output

The match type contains an illegal case:
    case Append[init, KV[k, v]] => KeysOf[init] | k
The pattern contains an unaccounted type parameter `k`.
(this error can be ignored for now with `-source:3.3`)

https://scastie.scala-lang.org/6ZDIroNFQQuOwgG78Ju83w

Expectation

Should compile.

[Gedochao]

Works correctly with LTS, including 3.3.5 and 3.3.6-RC1
Last good release: 3.4.0-RC1-bin-20231214-423cd6c-NIGHTLY
First bad release: 3.4.0-RC1-bin-20231218-ec2b8bc-NIGHTLY
The bisect script points at 90f9d22, but it fails with a different error (and can't be right, since it actually has been backported to the LTS, which works fine).

So the faulty commit is somewhere in between 90f9d22..ec2b8bc

[Gedochao]

Bisected it manually.
The faulty commit is 881e945 (#18262)
cc @sjrd

[sjrd]

It looks to be as designed per the spec, although the error message is misleading. k is "unaccounted for" because it is twice nested within contravariant type constructors.

The match type can be rewritten as follows to make it a legal match type:

type KeysOf[KVs] = KVs match
  case KV[k, v] => k
  case Append[init, kv] =>
    kv match
      case KV[k, v] => KeysOf[init] | k

Generally, only the deepest type constructor before type variables can be non-covariant. The 1-to-(n-1) outer type constructors must be covariant.

[Gedochao]

It looks to be as designed per the spec, although the error message is misleading.

@sjrd Should we try to improve the error message?

[sjrd]

Yes, it would be nice to have a better error message.