extractor for only possible concrete class of sealed trait considered refutable

[bishabosha]

Compiler version

3.3.1

Minimized code

sealed trait Foo
final case class FooImpl(x: Int) extends Foo

def someFoo: Foo = FooImpl(23)
val FooImpl(myInt) = someFoo

Output

-- Warning: --------------------------------------------------------------------
5 |val FooImpl(myInt) = someFoo
  |                     ^^^^^^^
  |pattern's type FooImpl is more specialized than the right hand side expression's type Foo
  |
  |If the narrowing is intentional, this can be communicated by adding `: @unchecked` after the expression,
  |which may result in a MatchError at runtime.
val myInt: Int = 23

Expectation

possibly be more sophisticated and accept when there is only one possible pattern?

[bishabosha]

this did come from a test case, so I imagine its quite rare to have real-life code that really only has one child of a sealed trait.

[armanbilge]

I just encountered this in http4s/http4s-crypto#179 :)

its quite rare to have real-life code that really only has one child of a sealed trait.

Perhaps. This strategy is useful to hide an implementation case class (that cannot be evolved binary-compatibly) behind a user-facing sealed trait. The codebase may internally prefer to operate on terms of the implementation class.

[bishabosha]

perfect, we have the real world use case excellently put!