Union type reduction fails in Tuple(A, A).toList

[yurique]

Compiler version

• 3.8.0-RC1-bin-20251007-71fcd1a-NIGHTLY
• 3.7.4-RC
• 3.7.3
• 3.7.2
• 3.6.4
• 3.5.2

This does NOT reproduce in 3.3.6 LTS.

Minimized code

Minimization from Aly in Discord:

//> using scala 3.8.0-RC1-bin-20251007-71fcd1a-NIGHTLY
def foo(x: (String, String)): List[String] = x.toList

(https://scastie.scala-lang.org/i0Zz6XF1RECi9EUzjklVFg)

Output

[error] ./tuple.sc:2:46
[error] Found:    List[Tuple.Union[(x : (String, String))]]
[error] Required: List[String]
[error]
[error] Note: a match type could not be fully reduced:
[error]
[error]   trying to reduce  Tuple.Union[(x : (String, String))]
[error]   trying to reduce  Tuple.Fold[(x : (String, String)), Nothing, [x, y] =>> x | y]
[error]   failed since selector (x : (String, String))
[error]   does not uniquely determine parameters h, t in
[error]     case h *: t => h | Tuple.Fold[t, Nothing, [x, y] =>> x | y]
[error]   The computed bounds for the parameters are:
[error]     h <: String
[error]     t <: String *: EmptyTuple.type
[error] def foo(x: (String, String)): List[String] = x.toList
[error]                                              ^^^^^^^^

---

Initially, we discovered this issue when seeing different behaviors with and without case

• .map { case x => x.toList } - compiles
• .map { x => x.toList } - does not compile

//> using scala 3.8.0-RC1-bin-20251007-71fcd1a-NIGHTLY

sealed trait Container[+A] { def map[B](fn: A => B) = ??? }

val tup: Container[(String, String)] = ???

// compiles
tup.map { case x => x.toList }

// does not compile
tup.map { x => x.toList }

(https://scastie.scala-lang.org/ISWA7siCTmqjJ9D2dUF2WQ)

Output:

[error] ./tuple2.sc:11:16
[error] Found:    List[Tuple.Union[(x : (String, String))]]
[error] Required: List[String]
[error]
[error] Note: a match type could not be fully reduced:
[error]
[error]   trying to reduce  Tuple.Union[(x : (String, String))]
[error]   trying to reduce  Tuple.Fold[(x : (String, String)), Nothing, [x, y] =>> x | y]
[error]   failed since selector (x : (String, String))
[error]   does not uniquely determine parameters h, t in
[error]     case h *: t => h | Tuple.Fold[t, Nothing, [x, y] =>> x | y]
[error]   The computed bounds for the parameters are:
[error]     h <: String
[error]     t <: String *: EmptyTuple.type
[error] tup.map { x => x.toList }
[error]                ^^^^^^^^

Expectation

The code should compile.