Test for non-open classes warning

Fixes #13946

Author: odersky

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

@@ -140,6 +140,7 @@ class CompilationTests {
       compileFilesInDir("tests/neg-custom-args/explicit-nulls", defaultOptions.and("-Yexplicit-nulls")),
       compileFilesInDir("tests/neg-custom-args/no-experimental", defaultOptions.and("-Yno-experimental")),
       compileDir("tests/neg-custom-args/impl-conv", defaultOptions.and("-Xfatal-warnings", "-feature")),
+      compileDir("tests/neg-custom-args/i13946", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFile("tests/neg-custom-args/implicit-conversions.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFile("tests/neg-custom-args/implicit-conversions-old.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFile("tests/neg-custom-args/i3246.scala", scala2CompatMode),

tests/neg-custom-args/i13946/BadPrinter.scala

@@ -0,0 +1,4 @@
+// in BadPrinter.scala
+import language.future
+class BadPrinter extends Printer: // error
+  override def print(s: String): Unit = println("Bad!!!")

tests/neg-custom-args/i13946/Printer.scala

@@ -0,0 +1,3 @@
+// in Printer.scala
+class Printer:
+  def print(s: String): Unit = println(s)