Allow experimental erased in experimental scopes

Closes #13392

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3115,7 +3115,7 @@ object Parsers {
         case Some(prefix) =>
           in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
           if prefix == nme.experimental
-             && selectors.exists(sel => Feature.experimental(sel.name) != Feature.scala2macros)
+             && selectors.exists(sel => Feature.experimental(sel.name) != Feature.scala2macros && Feature.experimental(sel.name) != Feature.erasedDefinitions)
           then
             Feature.checkExperimentalFeature("features", imp.srcPos)
           for

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

@@ -12,6 +12,8 @@ import typer.RefChecks
 import MegaPhase.MiniPhase
 import StdNames.nme
 import ast.tpd
+import SymUtils._
+import config.Feature
 
 /** This phase makes all erased term members of classes private so that they cannot
  *  conflict with non-erased members. This is needed so that subsequent phases like
@@ -39,13 +41,22 @@ class PruneErasedDefs extends MiniPhase with SymTransformer { thisTransform =>
     else cpy.Apply(tree)(tree.fun, tree.args.map(trivialErasedTree))
 
   override def transformValDef(tree: ValDef)(using Context): Tree =
+    checkErasedInExperimental(tree.symbol)
     if !tree.symbol.isEffectivelyErased || tree.rhs.isEmpty then tree
     else cpy.ValDef(tree)(rhs = trivialErasedTree(tree.rhs))
 
   override def transformDefDef(tree: DefDef)(using Context): Tree =
+    checkErasedInExperimental(tree.symbol)
     if !tree.symbol.isEffectivelyErased || tree.rhs.isEmpty then tree
     else cpy.DefDef(tree)(rhs = trivialErasedTree(tree.rhs))
 
+  override def transformTypeDef(tree: TypeDef)(using Context): Tree =
+    checkErasedInExperimental(tree.symbol)
+    tree
+
+  def checkErasedInExperimental(sym: Symbol)(using Context): Unit =
+    if sym.is(Erased) && sym != defn.Compiletime_erasedValue && !sym.isInExperimentalScope then
+      Feature.checkExperimentalFeature("erased", sym.sourcePos)
 }
 
 object PruneErasedDefs {

docs/docs/reference/experimental/erased-defs.md

@@ -9,6 +9,8 @@ title: "Erased Definitions"
 import scala.language.experimental.erasedDefinitions
 ```
 or by setting the command line option `-language:experimental.erasedDefinitions`.
+Erased definitions must be in an experimental scope (see [../other-new-features/experimental-defs.md]).
+
 ## Why erased terms?
 
 Let's describe the motivation behind erased terms with an example. In the

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

@@ -1,7 +1,9 @@
-import language.experimental.erasedDefinitions // error
+import language.experimental.erasedDefinitions
 import annotation.experimental
 
 @experimental
 erased class CanThrow[-E <: Exception]
 
+erased class CanThrow2[-E <: Exception] // error
+
 def other = 1

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

@@ -7,7 +7,7 @@ class Test0 {
 }
 
 class Test1 {
-  import scala.language.experimental.erasedDefinitions  // error
+  import scala.language.experimental.erasedDefinitions
   import scala.compiletime.erasedValue
   type UnivEq[A]
   object UnivEq:

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

@@ -0,0 +1,22 @@
+import language.experimental.erasedDefinitions
+import annotation.experimental
+
+@experimental
+erased class Foo
+
+erased class Bar // error
+
+@experimental
+erased def foo = 2
+
+erased def bar = 2 // error
+
+@experimental
+erased val foo2 = 2
+
+erased val bar2 = 2 // error
+
+@experimental
+def foo3(erased a: Int) = 2
+
+def bar3(erased a: Int) = 2 // error

tests/pos/experimentalErased.scala

@@ -0,0 +1,22 @@
+import language.experimental.erasedDefinitions
+import annotation.experimental
+
+@experimental
+erased class Foo
+
+erased class Bar
+
+@experimental
+erased def foo = 2
+
+erased def bar = 2
+
+@experimental
+erased val foo2 = 2
+
+erased val bar2 = 2
+
+@experimental
+def foo3(erased a: Int) = 2
+
+def bar3(erased a: Int) = 2

tests/pos/i13392.scala

@@ -0,0 +1,11 @@
+package scala
+import language.experimental.erasedDefinitions
+import annotation.{implicitNotFound, experimental}
+
+@experimental
+@implicitNotFound("The capability to throw exception ${E} is missing.\nThe capability can be provided by one of the following:\n - A using clause `(using CanThrow[${E}])`\n - A `throws` clause in a result type such as `X throws ${E}`\n - an enclosing `try` that catches ${E}")
+erased class CanThrow[-E <: Exception]
+
+@experimental
+object unsafeExceptions:
+  given canThrowAny: CanThrow[Exception] = ???