Traverse the types to find experimental references

Fixes #14034

Author: nicolasstucki

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

@@ -1339,7 +1339,18 @@ class RefChecks extends MiniPhase { thisPhase =>
   }
 
   override def transformTypeTree(tree: TypeTree)(using Context): TypeTree = {
-    checkExperimental(tree.symbol, tree.srcPos)
+    object CheckExperimental extends TypeTraverser {
+      def traverse(tp: Type): Unit =
+        tp match {
+          case tp: TypeRef =>
+            checkExperimental(tp.symbol, tree.srcPos)
+          case tp: TermRef =>
+            checkExperimental(tp.symbol, tree.srcPos)
+          case _ =>
+            traverseChildren(tp)
+        }
+    }
+    CheckExperimental.traverse(tree.tpe)
     tree
   }
 

tests/neg-custom-args/no-experimental/14034.scala

@@ -0,0 +1,12 @@
+import annotation.experimental
+
+@experimental trait Exp
+@experimental val exp = 1
+
+type Foo0 = Exp // error
+type Foo = Option[Exp] // error
+type Bar = Option[exp.type] // error
+type Baz = Exp | Int // error
+type Quux = [X] =>> X match // error
+  case Exp => Int
+type Quuz[A <: Exp] = Int // error