add/strict

src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -39,6 +39,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val defines = PrefixSetting("-Dproperty=value", "-D", "Pass -Dproperty=value directly to the runtime system.")
   val toolcp = PathSetting("-toolcp", "Add to the runner classpath.", "")
   val nobootcp = BooleanSetting("-nobootcp", "Do not use the boot classpath for the scala jars.")
+  val strict = BooleanSetting("-strict", "Use strict type rules, which means some formerly legal code does not typecheck anymore.")
 
   val argfiles = BooleanSetting("@<file>", "A text file containing compiler arguments (options and source files)")
   val classpath = PathSetting("-classpath", "Specify where to find user class files.", defaultClasspath) withAbbreviation "-cp"

src/dotty/tools/dotc/reporting/Reporter.scala

@@ -89,6 +89,10 @@ trait Reporting { this: Context =>
 
   def warning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new Warning(msg, pos))
+
+  def strictWarning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
+    if (this.settings.strict.value) error(msg, pos)
+    else warning(msg + "\n(This would be an error under strict mode)", pos)
 
   def error(msg: => String, pos: SourcePosition = NoSourcePosition): Unit = {
     // println("*** ERROR: " + msg) // !!! DEBUG

src/dotty/tools/dotc/typer/Applications.scala

@@ -716,7 +716,7 @@ trait Applications extends Compatibility { self: Typer =>
                   if (ctx.settings.verbose.value) ctx.warning(msg, tree.pos)
                 } else {
                   unapp.println(s" ${unapplyFn.symbol.owner} ${unapplyFn.symbol.owner is Scala2x}")
-                  ctx.error(msg, tree.pos)
+                  ctx.strictWarning(msg, tree.pos)
                 }
               case _ =>
             }