Do not require @experimental on anonymous classes

These calsses will not define new methods in the API for which we would need to check the presence of the annotation.

Fixes #13091

Author: nicolasstucki

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

@@ -1186,7 +1186,7 @@ object RefChecks {
 
   /** Check that classes extending experimental classes or nested in experimental classes have the @experimental annotation. */
   private def checkExperimentalInheritance(cls: ClassSymbol)(using Context): Unit =
-    if !cls.hasAnnotation(defn.ExperimentalAnnot) then
+    if !cls.isAnonymousClass && !cls.hasAnnotation(defn.ExperimentalAnnot) then
       cls.info.parents.find(_.typeSymbol.isExperimental) match
         case Some(parent) =>
           report.error(em"extension of experimental ${parent.typeSymbol} must have @experimental annotation", cls.srcPos)

tests/neg-custom-args/no-experimental/i13091.scala

@@ -0,0 +1,5 @@
+import annotation.experimental
+
+@experimental class Foo // error: use of @experimental is experimental ...
+
+def test: Unit = new Foo // error: class Foo is marked @experimental ...

tests/pos/i13091.scala

@@ -0,0 +1,3 @@
+import annotation.experimental
+@experimental class Foo
+val foo = new Foo