Unions of Singletons unreachable literals not being checked in pattern match.

The pattern match on the lhs side doesn’t exhaustively check if the pattern is of valid type for singletons of union.

In this example both 240 and 4H are not a valid subtype of both TimeframeN and Timeframe respectively, but compiler appears to compile without reporting type mismatch errors. It appears as if union type is widen’ed prematurely.

Compiler version

3.0.1-RC1

Minimized code

import scala.language.implicitConversions

type Timeframe = "1m" | "2m" | "1H"
type TimeframeN = 1 | 2 | 60

def manualConvertToN(tf: Timeframe): TimeframeN = tf match
  case "1m" => 1
  case "2m" => 2
  case "1H" => 60
  case "4H" => ??? // incorrect but compiles

given Conversion[Timeframe, TimeframeN] = 
  case "1m" => 1
  case "2m" => 2
  case "1H" => 60
  case "4H" => ??? // incorrect but compiles

given Conversion[TimeframeN, Timeframe] = 
  case 1 => "1m"
  case 2 => "2m"
  case 60 => "1H"
  case 240 => ??? // incorrect but compiles

Output

No errors on compilation

Expectation

Type mismatch errors. Hypothetical sample:

[error] -- [E007] Type Mismatch Error: /Users/swoorup.joshi/playground/test/src/main/scala/Main.scala:18:15
[error] 18 |  case "4H" => ???
[error]    |        ^^^
[error]    |        Found:    ("4H" : String)
[error]    |        Required: Timeframe

[error] -- [E007] Type Mismatch Error: /Users/swoorup.joshi/playground/test/src/main/scala/Main.scala:24:15
[error] 24 |  case "4H" => ???
[error]    |        ^^^
[error]    |        Found:    ("4H" : String)
[error]    |        Required: Timeframe

[error] -- [E007] Type Mismatch Error: /Users/swoorup.joshi/playground/test/src/main/scala/Main.scala:30:15
[error] 30 |  case 240 => ???
[error]    |       ^^^
[error]    |       Found:    (240 : Int)
[error]    |       Required: TimeframeN

Note that this doesn’t happen if the returning subtype is invalid. In such cases, compiler reports a type mismatch error correctly.

[SethTisue]

Dale and I attempted a fix on the draft PR https://github.com/lampepfl/dotty/pull/13290, but our efforts may have stalled; see our recent remarks there.

[romanowski]

It seems that exclusivity checks does not pick up a case where additional singletons are provided.

scala> def foo(a: 1 | 2) = a match       // Compiles fine                                                                                                               
     |   case 1 => true
     |   case 2 => false
     |   case 4 => ???                          
     | 
def foo(a: 1 | 2): Boolean

scala> def bar(a: 1 | 2) = a match                                                                                                                       
     |   case 1 => true
     |   case 2 => false
     |   case _ => ???
     | 
4 |  case _ => ???
  |       ^
  |       Unreachable case

scala> def baz(a: 1 | 2) = a match                                                                                                                       
     |   case 1 => true
     |   case 2 => false
     |   case a if a < 0 => ???
     | 
4 |  case a if a < 0 => ???
  |       ^
  |       Unreachable case