Fix ClassCastException upon class Foo extends Int => <someTerm>

[Blaisorblade]

To successfully typecheck

class Foo extends someTree

we need to detect correctly if someTree should be typechecked as a term or a
type.

If someTree is Int => 1, dotty crashes because typedFunction sees that Tree.isTerm on Int => this (or, untpd.Function(List('Int), 'this)) gives true and calls typedFunctionValue to typecheck this as a term. typedFunctionValue then crashes on an unchecked pattern match, because 'Int becomes an Ident and not a ValDef as it expects.

Instead, I think Int => 1 should be typechecked as a type, though I'm open to feedback.

Really, we shouldn't call isType at all, because the user might write a type in
place of a term or viceversa. I think we only want to know this is a constructor
call or a type; and maybe we should just let the parser tell us which, since it
knows.

Minimized from a testcase in #4389, in particular:

// 0f12f42d878dcd4cb8b8ab62dbfb6b41861989b1.scala
object i0 {
def main(i1: Array[String]): Unit = {
class i2
}
class i3(i4: => String) extends (i1 => (this
19): Option[String, Int] => 1
}

which triggers

Exception in thread "main" java.lang.ClassCastException: dotty.tools.dotc.ast.Trees$Ident cannot be cast to dotty.tools.dotc.ast.Trees$ValDef
	at dotty.tools.dotc.typer.Typer.$anonfun$11(Typer.scala:922)
	at scala.collection.TraversableLike$WithFilter.$anonfun$map$2(TraversableLike.scala:739)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.collection.TraversableLike$WithFilter.map(TraversableLike.scala:738)
	at dotty.tools.dotc.typer.Typer.typedFunctionValue(Typer.scala:926)
	at dotty.tools.dotc.typer.Typer.typedFunction(Typer.scala:756)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1790)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1832)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1819)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1832)
	at dotty.tools.dotc.typer.Typer.op1$2(Typer.scala:1863)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1859)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1871)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1931)
	at dotty.tools.dotc.typer.Typer.typedParent$2(Typer.scala:1491)
	at dotty.tools.dotc.typer.Typer.$anonfun$29(Typer.scala:1525)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:62)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:78)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1453)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1772)
[...]

[Blaisorblade]

Assigned to Martin as decided in the meeting.

[odersky]

I agree with the reasoning, but think we can cut it even more clearly. If you look at what the parser does (in constrApp), it produces either a type or a New expression applied to arguments. So I believe we can just check whether the parent is an Apply node and type it as a term, or type it as a type otherwise.

Can you try this and see whether it works?

[odersky]

@Blaisorblade Inactive for 6 months. What will happen with this?

Addressed comments

[Blaisorblade]

Last commit is from #5721 to allow tests to pass.

[biboudis]

I tested it with a partial functions and implicit function types and it works. They could be added as tests but this looks already good.