Compiler crash with annotation on main method

[lrytz]

//> using scala 3.6.2

import util.chaining.*

class ann(x: Int = 1, y: Int) extends annotation.Annotation

@ann(y = 22.tap(println)) @main def main = ()

Error: scala.MatchError: Block(List(ValDef(y$1,TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Int)],Apply(TypeApply(Select(Apply(TypeApply(TypedSplice(Ident(scalaUtilChainingOps)),List(TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Int)])),List(Literal(Constant(22)))),tap),List(TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Unit)])),List(Block(List(DefDef($anonfun,List(List(ValDef(x,TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Any)],EmptyTree))),TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class Unit)],Apply(TypedSplice(Ident(println)),List(TypedSplice(Ident(x)))))),Closure(List(),TypedSplice(Ident($anonfun)),EmptyTree))))), ValDef(x$1,TypeTree[AnnotatedType(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int),ConcreteAnnotation(Apply(Select(New(TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class unchecked)),class uncheckedVariance)]),<init>),List())))],Select(TypedSplice(Ident(ann)),$lessinit$greater$default$1))),Apply(Select(New(TypedSplice(Ident(ann))),<init>),List(TypedSplice(Ident(x$1)), NamedArg(y,TypedSplice(Ident(y$1)))))) (of class dotty.tools.dotc.ast.Trees$Block)
	at dotty.tools.dotc.typer.Namer.typedAheadAnnotationClass(Namer.scala:1774)
	at dotty.tools.dotc.typer.Namer$Completer.addAnnotations$$anonfun$1(Namer.scala:883)

[mbovel]

Not a crash anymore (probably following my fixes for default arguments):

bovel@laraserver4:/localhome/bovel/scala3$ scala -S 3.7.nightly tests/pos/22364.scala
Downloading Scala 3.7.0-RC1-bin-20250207-d60a914-NIGHTLY compiler
Downloading Scala 3.7.0-RC1-bin-20250207-d60a914-NIGHTLY bridge
Compiling project (Scala 3.7.0-RC1-bin-20250207-d60a914-NIGHTLY, JVM (17))
[error] ./tests/pos/22364.scala:5:10
[error] cannot infer type; expected type <?> is not fully defined
[error] @ann(y = 22.tap(println)) @main def main = ()
[error]          ^^^^^^
Error compiling project (Scala 3.7.0-RC1-bin-20250207-d60a914-NIGHTLY, JVM (17))
Compilation failed

But it should also not be an error.

[mbovel]

The essential ingredients here seem to be @main and a type parameter:

def id[T](x: T): T = x

class ann(x: Int) extends annotation.Annotation

@ann(id(22)) @main def main = ()

Which becomes:

sbt:scala3> scalac -Xprint:typer tests/pos/22364.scala
...
-- Error: tests/pos/22364.scala:5:5 --------------------------------------------
5 |@ann(id(22)) @main def main = ()
  |     ^^
  |     cannot infer type; expected type <?> is not fully defined
[[syntax trees at end of                     typer]] // tests/pos/22364.scala
package <empty> {
  class ann(x: Int) extends annotation.Annotation() {
    private[this] val x: Int
  }
  final lazy module val 22364$package: 22364$package = new 22364$package()
  final module class 22364$package() extends Object() {
    this: 22364$package.type =>
    def id[T >: Nothing <: Any](x: T): T = x
    @main @ann(id[Int](22)) def main: Unit = ()
  }
  final class main() extends Object() {
    @ann(
      id[<error cannot infer type; expected type <?> is not fully defined>](22))
       <static> def main(args: Array[String]): Unit =
      try main catch 
        {
          case error @ _:scala.util.CommandLineParser.ParseError =>
            scala.util.CommandLineParser.showError(error)
        }
  }
}

The same example without @main type-checks:

def id[T](x: T): T = x

class ann(x: Int) extends annotation.Annotation

@ann(id(22)) @ann(id(22)) def main = ()

[mbovel]

The root problem is how we copy annotations when we generate the @main proxy in

scala3/compiler/src/dotty/tools/dotc/ast/MainProxies.scala

Lines 101 to 111 in 049308f

	/** Replace typed `Ident`s that have been typed with a TypeSplice with the reference to the symbol.
	* The annotations will be retype-checked in another scope that may not have the same imports.
	*/
	def insertTypeSplices = new TreeMap {
	override def transform(tree: Tree)(using Context): Tree = tree match
	case tree: tpd.Ident @unchecked => TypedSplice(tree)
	case tree => super.transform(tree)
	}
	val annots = mainFun.annotations
	.filterNot(_.matches(defn.MainAnnot))
	.map(annot => insertTypeSplices.transform(annot.tree))

This logic was introduced in #13858.