Special case experimental in Predef additions

Author: odersky

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -276,6 +276,9 @@ object Annotations {
       }
   }
 
+  /** This annotation is added by the compiler to all top-level definitions that
+   *  are compiled in experimental mode.
+   */
   object ExperimentalAnnotation {
 
     /** Create an instance of `@experimental(<msg>)` */
@@ -301,4 +304,14 @@ object Annotations {
       }
   }
 
+  /** This annotation is added by the compiler to all experimental definitions
+   *  in stdLibPatches, where adding an explicit @experimental would lead to a cycle.
+   */
+  class LazyExperimentalAnnotation(span: Span) extends LazyAnnotation:
+    protected var mySym =
+      (ctx: Context) ?=> defn.ExperimentalAnnot
+
+    protected var myTree =
+      (ctx: Context) ?=> New(defn.ExperimentalAnnot.typeRef, Literal(Constant("")) :: Nil).withSpan(span)
+  end LazyExperimentalAnnotation
 }

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1461,6 +1461,10 @@ class Definitions {
                 makeNonClassSymbol(patch)
               end match
             sym.annotations = patch.annotations
+            if experimentalInPredef.contains(patch.name)
+                && patchCls.name == tpnme.Predef.moduleClassName
+            then
+              sym.addAnnotation(Annotations.LazyExperimentalAnnotation(sym.span))
             scope.enter(sym)
           if patch.isClass then
             patch2(scope.lookup(patch.name).asClass, patch)
@@ -1477,6 +1481,12 @@ class Definitions {
       patchWith(LanguageModuleClassPatch)
   end patchStdLibClass
 
+  /** Definitions that are assumed to be experimental in Predef.
+   *  We can't mark them with @experimental since that causes a cycle.
+   *  So we add the annotation instead when we patch Predef.
+   */
+  private val experimentalInPredef: Set[Name] = Set("is".toTypeName)
+
   // ----- Symbol sets ---------------------------------------------------
 
   @tu lazy val topClasses: Set[Symbol] = Set(AnyClass, MatchableClass, ObjectClass, AnyValClass)

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -72,8 +72,9 @@ object Predef:
    *    TC { type Self = A }
    *
    *  which is what is needed for a context bound `[A: TC]`.
+   *
+   *  @experimental added by compiler in Definitions.patchStdLibClass
    */
-  @experimental
   infix type is[A <: AnyKind, B <: Any{type Self <: AnyKind}] = B { type Self = A }
 
 end Predef

tests/neg/experimentalInPredef.scala

@@ -0,0 +1,6 @@
+//> using options -Yno-experimental
+class C:
+  type Self
+
+def Test[T](witness: T is C) = () // error
+

tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -81,7 +81,6 @@ val experimentalDefinitionInLibrary = Set(
   "scala.Precise",
   "scala.annotation.internal.WitnessNames",
   "scala.compiletime.package$package$.deferred",
-  "scala.runtime.stdLibPatches.Predef$.is",
 )