Pattern match with Singleton type failed

Question
1

The following codes (see the two ‘ERR’ lines below) cannot be compiled using Scala 3.3.0-RC3

type TestMatchType[T] = T match
  case "String1" => Int
  case "String2" => String

def testMatchType[T](input: T): TestMatchType[input.type]  =
  input match
    case "String1": "String1" => 1        // ERR: '=>' expected, but ':' found
    case "String2": "String2" => "Two"    // ERR: '=>' expected, but ':' found

How to fix them?

Thanks for your help!

Guofeng

2

Worked out with

def testMatchType[T](input: T): TestMatchType[input.type]  =
  input match
    case _: "String1" => 1
    case _: "String2" => "Two"
3

Basically scala has two types of patterns (can’t recall the proper names for them though):

  1. “variable patterns”, e.g.
    case x => ...
    case x: Int => ...
  2. “value/extractor patterns”, e.g.
    case 1 => ...
    case Some(1) => ...
    case x @ Some(1) => ...

Note that only variable patterns can have type ascriptions (like : Int) and only value/extractor patterns can have name bindings (like x @).
Extractor patterns can have other patterns embedded in them, e.g. case Some(x: Int) => but this doesn’t.
Before it happened to be possible to write things like case "abc": "abc" => ... but it caused some syntactic problems and didn’t seem to have any semantic benefits as in general checking the type and checking the value could be performed separately.
Match types are a feature which still isn’t very popular so there still might be some corner cases which could be handled in a smoother way (and combining match types with matches on values is one of them).

4

@chuwy’s solution solves the issue.

Thanks all.