add/strict

Add -strict option to  do some type checks that are encessary to ensure type soundness, but
are stricter than what Scala 2.x enforces.

The first such test is the "pattern cannot be uniquely instantiated" problem where we reject a
non-variant case subclass of a covariant superclass in a pattern match. The error is now only
issued in -struct mode, otherwise it will be a warning.

We might move more tests into the same category. This should help the transition.

Author: odersky

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 _ =>
             }