Remove restriction on val _ = .. with top-level experimental imports

Author: nicolasstucki

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

@@ -843,9 +843,8 @@ object Checking {
               val sym = tree.symbol
               if !sym.isExperimental then
                 sym.addAnnotation(ExperimentalAnnotation(s"Added by $why", sym.span))
-            case tree =>
-              // There is no definition to attach the @experimental annotation
-              report.error(s"Implementation restriction: top-level `val _ = ...` is not supported with $why.", tree.srcPos)
+            case _ =>
+              // statements from a `val _ = ...`
       unitExperimentalLanguageImports match
         case imp :: _ => markTopLevelDefsAsExperimental(i"top level $imp")
         case _ =>

tests/neg/experimental-import-with-top-level-val-underscore.check

@@ -18,7 +18,7 @@ def handle[E <: Exception, R](op: (erased CanThrow[E]) -> R)(handler: E -> R): R
   try op(x)
   catch case ex: E => handler(ex)
 
-val bar = handle { (erased x) =>
+val _ = handle { (erased x) =>
     if true then
       raise(new Exception)(using x)
       22

tests/pos-custom-args/captures/try.scala

@@ -1,6 +1,8 @@
 
 import language.experimental.erasedDefinitions
 
-val _ = // error
-  println("Hello, world!")
+def test() = ()
+
+val _ =
+  test()
   42