Pattern matching against enum case without parameters compiles without warnings

[lolgab]

Compiler version

3.5.0 (3.4.0+)
3.3.3 works correctly

Minimized code

enum Foo:
  case Bar(i: Int)
case class Container(foo: Foo) 

@main def main = Container(Foo.Bar(1)) match {
  case Container(Foo.Bar) => println("yes")
  case _ => println("no")
}

Output

Using Scala 3.5.0

no

Using Scala 3.3.3

[error] ./main.scala:8:18
[error] Found:    Foo.Bar.type
[error] Required: Foo
[error] pattern type is incompatible with expected type
[error]   case Container(Foo.Bar) => println("yes")
[error]                  ^^^^^^^
Error compiling project (Scala 3.3.3, JVM (11))
Compilation failed

Expectation

It should fail to compile since Container takes a Foo and I expect Foo.Bar to be of type Foo.Bar.type but not of type Foo.
Scala 3.3.3 does the right thing, but 3.5.0 doesn't.

[bracevac]

I think these kinds of checks are now covered by -Wimplausible-patterns since 3.4. See #18093.

[Gedochao]

BTW, the error is still raised in 3.3.4-RC4.

I think these kinds of checks are now covered by -Wimplausible-patterns since 3.4. See #18093.

@bracevac it's true -Wimplausible-patterns correctly emits a warning for 3.4+

[warn] ./repro.scala:7:18
[warn] Implausible pattern:
[warn] Foo.Bar  could match selector of type  Foo
[warn] only if there is an `equals` method identifying elements of the two types.
[warn]   case Container(Foo.Bar) => println("yes")
[warn]                  ^^^^^^^

So at first glance it looks like a regression, but indeed it seems these checks have been moved behind a linting flag and the errors downgraded to warnings. I wonder if it wouldn't make sense to be able to identify them as errors (I'd personally expect them to be errors).
However, perhaps it'd be enough to document this better...

cc @odersky @dwijnand

[odersky]

If I add an equals method like this

enum Foo:
  def equals(x: Bar, y: Bar) = true
  case Bar(i: Int)
case class Container(foo: Foo)

@main def main = Container(Foo.Bar(1)) match
  case Container(Foo.Bar) => println("yes")
  case _ => println("no")

I still get a warning. So I think should not be an error, because now the program would print "yes". The problem is the compiler does not know what the equals method, if one is given, does. So it has to argue on the basis of plausibility, and such guesses should in general not be errors.

[Gedochao]

@odersky ah, I see, thanks for explaining.
Then, even if confusing, I guess the current behaviour is expected.

[lolgab]

One thing I don't understand is the role of the Bar term. What is it? The equivalent of a companion object for a case class? Why can be used interchangeably with the instances of Bar?
For sure I'm missing a lot of how enums are represented.

[OlegYch]

If I add an equals method like this

enum Foo:
  def equals(x: Bar, y: Bar) = true
  case Bar(i: Int)
case class Container(foo: Foo)

@main def main = Container(Foo.Bar(1)) match
  case Container(Foo.Bar) => println("yes")
  case _ => println("no")

I still get a warning. So I think should not be an error, because now the program would print "yes". The problem is the compiler does not know what the equals method, if one is given, does. So it has to argue on the basis of plausibility, and such guesses should in general not be errors.

There is no way Foo.Bar and Foo.Bar(1) can be equal, so the attempted comparison should fail with compilation error.

[odersky]

Yes but the example can easily be changed so that's no longer true.

[OlegYch]

@odersky that would be a separate issue