Fix experimental in signature check

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -15,7 +15,6 @@ import ContextFunctionResults.annotateContextResults
 import config.Printers.typr
 import reporting._
 
-
 object PostTyper {
   val name: String = "posttyper"
 }

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -978,6 +978,16 @@ object RefChecks {
     then
       Feature.checkExperimentalDef(sym, pos)
 
+  private def checkExperimentalTypes(tpe: Type, pos: SrcPos)(using Context): Unit =
+    val checker = new TypeTraverser:
+      def traverse(tp: Type): Unit =
+        if tp.typeSymbol.isExperimental then
+          Feature.checkExperimentalDef(tp.typeSymbol, pos)
+        else
+          traverseChildren(tp)
+    if !pos.span.isSynthetic then // avoid double errors
+      checker.traverse(tpe)
+
   private def checkExperimentalAnnots(sym: Symbol)(using Context): Unit =
     for annot <- sym.annotations if annot.symbol.isExperimental do
       Feature.checkExperimentalDef(annot.symbol, annot.tree)
@@ -1220,6 +1230,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     checkNoPrivateOverrides(tree)
     checkDeprecatedOvers(tree)
     checkExperimentalAnnots(tree.symbol)
+    checkExperimentalTypes(tree.symbol.info, tree)
     val sym = tree.symbol
     if (sym.exists && sym.owner.isTerm) {
       tree.rhs match {
@@ -1241,6 +1252,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     checkNoPrivateOverrides(tree)
     checkDeprecatedOvers(tree)
     checkExperimentalAnnots(tree.symbol)
+    checkExperimentalTypes(tree.symbol.info, tree)
     checkImplicitNotFoundAnnotation.defDef(tree.symbol.denot)
     tree
   }

tests/neg-custom-args/no-experimental/experimentalType.scala

@@ -15,9 +15,11 @@ type Y = Int // error
 @experimental
 opaque type Z = Int // error
 
+type W = Z // error
+
 def test(
   p1: A, // error
-  p2: List[A], // FIXME ERROR
+  p2: List[A], // error
   p3: X, // error
   p4: Y, // error
   p5: Z, // error

tests/neg-custom-args/no-experimental/experimentalUnapply.scala

@@ -4,7 +4,7 @@ import scala.annotation.experimental
 class A
 
 object Extractor1:
-  def unapply(s: Any): Option[A] = ??? // FIXME ERROR
+  def unapply(s: Any): Option[A] = ??? // error
 
 object Extractor2:
   @experimental // error