Union singleton types weirdness with isInstanceOf

[Swoorup]

isInstanceOf has difference when run in compile time vs runtime. Here is an sbt example where a valid subtype isn't detected, when passed as method arg.

Compiler version

3.0.1-RC1

Minimized code

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

@main def main(input: String) = 
  val isTimeframe = input.isInstanceOf[Timeframe]
  val oneM: String = "1m"
  println(s"input is of Timeframe: $isTimeframe")
  println(s"1m is of Timeframe: ${oneM.isInstanceOf[Timeframe]}")

Output

Running with 1m which is a valid Timeframe type.

sbt:scala3-simple> run 1m
[info] running main 1m
input is of Timeframe: false
1m is of Timeframe: true

Expectation

sbt:scala3-simple> run 1m
[info] running main 1m
input is of Timeframe: true
1m is of Timeframe: true

Original motivation for doing this was to check if an input from untrusted source is of the specified type.

[Swoorup]

So somehow the compiler also knows about literals in the code being at the callsite, when you write code like this

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

def ok(input: String) = 
  val oneMinute: String = "1m"
  println(s"input: $input is of Timeframe: ${input.isInstanceOf[Timeframe]}")
  println(s"1m is of Timeframe: ${oneMinute.isInstanceOf[Timeframe]}")

@main def main(input: String) = 
  ok("1H")
  ok(input)

This is also a bit troubling.

sbt:scala3-simple> run 1m
input: 1H is of Timeframe: true
1m is of Timeframe: true
input: 1m is of Timeframe: false
1m is of Timeframe: true