Do not accidentally force diagnostic message in reportWarning

Diagnostic messages are contained within a `MessageContainer` which
takes the message by name. This is in order for the compiler to be able
to throw away messages without evaluting potentially costly operations.
Calling `.contained` breaks this laziness.

Author: felixmulder

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

@@ -37,8 +37,8 @@ trait Reporting { this: Context =>
   def echo(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new Info(msg, pos))
 
-  def reportWarning(warning:Warning):Unit =
-    if(this.settings.XfatalWarnings.value) error(warning.contained, warning.pos)
+  def reportWarning(warning: Warning): Unit =
+    if (this.settings.XfatalWarnings.value) reporter.report(warning.toError)
     else reporter.report(warning)
 
   def deprecationWarning(msg: => Message, pos: SourcePosition = NoSourcePosition): Unit =

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -31,7 +31,9 @@ object messages {
   class Warning(
     msgFn: => Message,
     pos: SourcePosition
-  ) extends MessageContainer(msgFn, pos, WARNING)
+  ) extends MessageContainer(msgFn, pos, WARNING) {
+    def toError: Error = new Error(msgFn, pos)
+  }
 
   class Info(
     msgFn: => Message,