Drop experimental inheritance checking

IMO it's a foot gun that prevents useful idioms. Concretely I have an @experimental marker
trait `@caps.Pure` that classes can inherit. So far, there was no way to do that except
if the inheriting class is also @experimental. This requirement was unconditional. It
did not help to be in an experimental scope or to use a snapshot compiler version.

This basically means that @experimental base traits are useless, unless one is prepared to
pollute the codebase with sprinkling experimental over all inheriting classes.

On the other hand, I see no useful property that is established by this restriction. The (now
deleted) documentation is also silent about the motivation of all this.

Author: odersky

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

@@ -115,15 +115,6 @@ class CrossVersionChecks extends MiniPhase:
     }
   }
 
-  /** 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.isAnonymousClass && !cls.isInExperimentalScope 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)
-        case _ =>
-  end checkExperimentalInheritance
-
   override def transformValDef(tree: ValDef)(using Context): ValDef =
     checkDeprecatedOvers(tree)
     checkExperimentalAnnots(tree.symbol)
@@ -136,12 +127,6 @@ class CrossVersionChecks extends MiniPhase:
     checkExperimentalSignature(tree.symbol, tree)
     tree
 
-  override def transformTemplate(tree: Template)(using Context): Tree =
-    val cls = ctx.owner.asClass
-    checkExperimentalInheritance(cls)
-    checkExperimentalAnnots(cls)
-    tree
-
   override def transformIdent(tree: Ident)(using Context): Ident = {
     checkUndesiredProperties(tree.symbol, tree.srcPos)
     tree

docs/_docs/reference/other-new-features/experimental-defs.md

@@ -216,7 +216,7 @@ Experimental definitions can only be referenced in an experimental scope. Experi
 
    <details>
    <summary>Example 1</summary>
-   
+
    ```scala
    import scala.annotation.experimental
 
@@ -242,7 +242,7 @@ Experimental definitions can only be referenced in an experimental scope. Experi
      }
    }
    ```
-   
+
    </details>
 
 5. Annotations of an experimental definition are in experimental scopes. Examples:
@@ -270,13 +270,6 @@ Can use the `-Yno-experimental` compiler flag to disable it and run as a proper
 
 In any other situation, a reference to an experimental definition will cause a compilation error.
 
-## Experimental inheritance
-
-All subclasses of an experimental `class` or `trait` must be marked as [`@experimental`](https://scala-lang.org/api/3.x/scala/annotation/experimental.html) even if they are in an experimental scope.
-Anonymous classes and SAMs of experimental classes are considered experimental.
-
-We require explicit annotations to make sure we do not have completion or cycles issues with nested classes. This restriction could be relaxed in the future.
-
 ## Experimental overriding
 
 For an overriding member `M` and overridden member `O`, if `O` is non-experimental then `M` must be non-experimental.