Allow experimental language imports in experimental scopes

An experimental language import is permitted of the enclosing
scope is an experimental scopes, or if all statements in that
scope are @experimental definitions.

Author: odersky

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

@@ -13,6 +13,7 @@ import Decorators._
 import Symbols._, SymUtils._, NameOps._
 import ContextFunctionResults.annotateContextResults
 import config.Printers.typr
+import config.Feature
 import reporting._
 
 object PostTyper {
@@ -448,6 +449,31 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           throw ex
       }
 
+    /** In addition to normal processing, check that experimental language
+     *  imports are done only in experimental scopes, or in scopes where
+     *  all statements are @experimental definitions.
+     */
+    override def transformStats(trees: List[Tree], exprOwner: Symbol)(using Context): List[Tree] =
+
+      def onlyExperimentalDefs = trees.forall {
+        case _: Import | EmptyTree => true
+        case stat: MemberDef => stat.symbol.isExperimental
+        case _ => false
+      }
+
+      def checkExperimentalImports =
+        for case imp @ Import(qual, selectors) <- trees do
+          languageImport(qual) match
+            case Some(nme.experimental)
+            if !ctx.owner.isInExperimentalScope && !onlyExperimentalDefs
+               && selectors.exists(sel => Feature.experimental(sel.name) != Feature.scala2macros) =>
+              Feature.checkExperimentalFeature("features", imp.srcPos)
+            case _ =>
+
+      try super.transformStats(trees, exprOwner)
+      finally checkExperimentalImports
+    end transformStats
+
     /** Transforms the rhs tree into a its default tree if it is in an `erased` val/def.
      *  Performed to shrink the tree that is known to be erased later.
      */

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -178,7 +178,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/matchable.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/i7314.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/feature-shadowing.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
-      compileDir("tests/neg-custom-args/hidden-type-errors",  defaultOptions.and("-explain")),
+      compileDir("tests/neg-custom-args/hidden-type-errors", defaultOptions.and("-explain")),
     ).checkExpectedErrors()
   }
 

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

@@ -25,10 +25,4 @@ class Test2 {
 
 class Test6 {
   import scala.language.experimental  // ok
-}
-
-class Test7 {
-  import scala.language.experimental
-  import experimental.genericNumberLiterals // error: no aliases can be used to refer to a language import
-  val x: BigInt = 13232202002020202020202   // error
 }