Add warning for synchronized calls in value classes

Co-Authored-By: Yoonjae Jeon <18438185+nox213@users.noreply.github.com>

Author: mbovel

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

@@ -1239,7 +1239,9 @@ object RefChecks {
   def checkAnyRefMethodCall(tree: Tree)(using Context) =
     if tree.symbol.exists
        && defn.topClasses.contains(tree.symbol.owner)
-       && (!ctx.owner.enclosingClass.exists || ctx.owner.enclosingClass.isPackageObject) then
+       && (!ctx.owner.enclosingClass.exists
+           || ctx.owner.enclosingClass.isPackageObject
+           || ctx.owner.enclosingClass.isValueClass) then
       report.warning(UnqualifiedCallToAnyRefMethod(tree, tree.symbol), tree)
 
 }

tests/warn/i17493.check

@@ -0,0 +1,11 @@
+-- [E181] Potential Issue Warning: tests/warn/i17493.scala:3:11 --------------------------------------------------------
+3 |   def g = synchronized { println("hello, world") } // warn
+  |           ^^^^^^^^^^^^
+  |           Suspicious top-level unqualified call to synchronized
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Top-level unqualified calls to AnyRef or Any methods such as synchronized are
+  | resolved to calls on Predef or on imported methods. This might not be what
+  | you intended.
+   ---------------------------------------------------------------------------------------------------------------------

tests/warn/i17493.scala

@@ -0,0 +1,4 @@
+//> using options -explain
+ class A(val s: String) extends AnyVal {
+   def g = synchronized { println("hello, world") } // warn
+ }