("A" | "B") <:< Singleton ?

[armanbilge]

Compiler version

3.1.2

Minimized code

Welcome to Scala 3.1.2 (11.0.13, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                                                 
scala> summon[("A" | "B") <:< Singleton]
val res0: ("A" | "B") =:= ("A" | "B") = generalized constraint
                                                                                                                                                                 
scala> summon[ValueOf["A" | "B"]]
-- Error: -------------------------------------------------------------------------------------------------------------------------------------------------------
1 |summon[ValueOf["A" | "B"]]
  |                          ^
  |                          No singleton value available for ("A" : String) | ("B" : String).
1 error found
                                                                                                                                                                 
scala> 

Expectation

Seems strange that there is an instance of ("A" | "B") <:< Singleton. I was mighty curious what ValueOf might return!

[smarter]

Seems strange that there is an instance of ("A" | "B") <:< Singleton

It's inevitable because "A" <: Singleton and "B" <: Singleton, this is a known design flaw of Singleton (which predates union types) and eventually we'll have to replace it with some other mechanism, see #4944.