Confusion between syntax for pattern alternative and typed pattern on a union type

[abgruszecki]

As in the title, running this code in REPL throws java.lang.VerifyError:

scala> def f(): String | Int = 0
def f(): String | Int
scala> f() match { case s: String | Int => s }

java.lang.VerifyError: Bad local variable type
Exception Details:
  Location:
    rs$line$2$.<init>()V @33: aload_3
  Reason:
    Type top (current frame, locals[3]) is not assignable to reference type
  Current Frame:
    bci: @33
    flags: { }
    locals: { 'rs$line$2$', 'java/lang/Object' }
    stack: { 'rs$line$2$' }
  Bytecode:
    0x0000000: 2ab7 0014 2ab3 0016 2ab2 001b b600 1f4c
    0x0000010: 2bc1 0021 9900 172b c000 214d 2c4e a700
    0x0000020: 072d a700 06a7 fffc a700 1cb2 0026 2bb6
    0x0000030: 002a 9900 09a7 ffec 0000 bfbb 002c 592b
    0x0000040: b700 2fbf b500 31b1
  Stackmap Table:
    full_frame(@33,{Object[#2],Object[#4]},{Object[#2]})
    full_frame(@37,{Object[#2],Object[#4],Object[#33],Object[#33]},{Object[#2]})
    full_frame(@40,{Object[#2],Object[#4]},{Object[#2],Top})
    same_locals_1_stack_item_frame(@43,Object[#2])
    full_frame(@56,{},{Object[#53]})
    full_frame(@59,{Object[#2],Object[#4]},{Object[#2]})
    full_frame(@68,{Object[#2],Object[#4]},{Object[#2],Top})

	at rs$line$2.res0Show(rs$line$2)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at dotty.tools.repl.Rendering.valueOf(Rendering.scala:58)
	at dotty.tools.repl.Rendering.renderVal(Rendering.scala:79)
	at dotty.tools.repl.ReplDriver.$anonfun$displayDefinitions$8(ReplDriver.scala:269)
	at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at scala.collection.TraversableLike.map(TraversableLike.scala:234)
	at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
	at scala.collection.AbstractTraversable.map(Traversable.scala:104)
	at dotty.tools.repl.ReplDriver.displayMembers$1(ReplDriver.scala:269)
	at dotty.tools.repl.ReplDriver.$anonfun$displayDefinitions$15(ReplDriver.scala:296)
	at scala.Option.map(Option.scala:146)
	at dotty.tools.repl.ReplDriver.$anonfun$displayDefinitions$13(ReplDriver.scala:294)
	at dotty.tools.dotc.core.Periods.atPhase(Periods.scala:26)
	at dotty.tools.dotc.core.Phases.atPhase(Phases.scala:36)
	at dotty.tools.dotc.core.Phases.atPhase$(Phases.scala:35)
	at dotty.tools.dotc.core.Contexts$Context.atPhase(Contexts.scala:57)
	at dotty.tools.repl.ReplDriver.displayDefinitions(ReplDriver.scala:289)
	at dotty.tools.repl.ReplDriver.$anonfun$compile$2(ReplDriver.scala:226)
	at scala.util.Either.fold(Either.scala:188)
	at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:220)
	at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:190)
	at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:143)
	at dotty.tools.repl.Main$.main(Main.scala:6)
	at dotty.tools.repl.Main.main(Main.scala)

Parenthesising the type, however, runs normally:

scala> f() match { case s: (String | Int) => s }
val res2: Any = 0

[allanrenucci]

Without parenthesis, it is parsed as

case (s @ _: String) | Int => 

with parenthesis:

case s @ _: String | Int =>

[allanrenucci]

But I think it is more than a parsing issue, the following code snippet compiles with Dotty and crashes at runtime:

object Main {
  def main(args: Array[String]): Unit = {
    (1: Any) match { case (s: String) | _: Int => s }
  }
}

Exception in thread "main" java.lang.VerifyError: Bad local variable type
Exception Details:
  Location:
    Main$.main([Ljava/lang/String;)V @23: aload
  Reason:
    Type top (current frame, locals[4]) is not assignable to reference type
  Current Frame:
    bci: @23
    flags: { }
    locals: { 'Main$', '[Ljava/lang/String;', 'java/lang/Integer' }
    stack: { }
  Bytecode:
    0x0000000: 04b8 0018 4d2c c100 1a99 001a 2cc0 001a
    0x0000010: 4e2d 3a04 a700 0919 0457 a700 06a7 fffa
    0x0000020: a700 1cb2 001f 2cb6 0023 9900 09a7 ffea
    0x0000030: 0000 bfbb 0025 592c b700 28bf b1       
  Stackmap Table:
    append_frame(@23,Object[#46])
    append_frame(@29,Object[#26],Object[#26])
    chop_frame(@32,2)
    same_frame(@35)
    full_frame(@48,{},{Object[#48]})
    append_frame(@51,Object[#2],Object[#49],Object[#46])
    same_frame(@60)

	at Main.main(Test.scala)

[smarter]

This code should not compile, as in scalac where you get:

try/i3332.scala:3: error: illegal variable in pattern alternative
    (1: Any) match { case (s: String) | _: Int => s }
                           ^

There's already an issue for that: #1612

The other issue is how to parse case s: String | Int, it's unfortunate that the current meaning is confusing when combined with union types, but I don't know if there's a good solution here