Try harder to avoid reporting unpositioned errors

Rather than issuing an error at NoPosition, which usually means
an unpositioned tree is being typechecked, walk up the context
chain for an enclosing positioned tree.

I've tested this manually with ensime which was getting an
unpositioned warning as a result of a unpositioned trees created
by a macro in scalatest.

```
% ../scala-positioned-error/build/quick/bin/scalac  @args.txt
/Users/jason/code/ensime-server/core/src/it/scala/org/ensime/core/RichPresentationCompilerSpec.scala:216: warning: implicit numeric widening
    ) { (p, cc) =>
                ^
```

1 files changed, 12 insertions, 8 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index 43f2655311..c46bc7444a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -574,19 +574,23 @@ trait Contexts { self: Analyzer =>
     /** Issue/buffer/throw the given implicit ambiguity error according to the current mode for error reporting. */
     private[typechecker] def issueAmbiguousError(err: AbsAmbiguousTypeError) = reporter.issueAmbiguousError(err)(this)
     /** Issue/throw the given error message according to the current mode for error reporting. */
-    def error(pos: Position, msg: String)                                    = reporter.error(pos, msg)
+    def error(pos: Position, msg: String)                                    = reporter.error(fixPosition(pos), msg)
     /** Issue/throw the given error message according to the current mode for error reporting. */
-    def warning(pos: Position, msg: String)                                  = reporter.warning(pos, msg)
-    def echo(pos: Position, msg: String)                                     = reporter.echo(pos, msg)
+    def warning(pos: Position, msg: String)                                  = reporter.warning(fixPosition(pos), msg)
+    def echo(pos: Position, msg: String)                                     = reporter.echo(fixPosition(pos), msg)
+    def fixPosition(pos: Position): Position = pos match {
+      case NoPosition => nextEnclosing(_.tree.pos != NoPosition).tree.pos
+      case _ => pos
+    }
 
 
     def deprecationWarning(pos: Position, sym: Symbol, msg: String): Unit =
-      currentRun.reporting.deprecationWarning(pos, sym, msg)
+      currentRun.reporting.deprecationWarning(fixPosition(pos), sym, msg)
     def deprecationWarning(pos: Position, sym: Symbol): Unit =
-      currentRun.reporting.deprecationWarning(pos, sym) // TODO: allow this to escalate to an error, and implicit search will ignore deprecated implicits
+      currentRun.reporting.deprecationWarning(fixPosition(pos), sym) // TODO: allow this to escalate to an error, and implicit search will ignore deprecated implicits
 
     def featureWarning(pos: Position, featureName: String, featureDesc: String, featureTrait: Symbol, construct: => String = "", required: Boolean): Unit =
-      currentRun.reporting.featureWarning(pos, featureName, featureDesc, featureTrait, construct, required)
+      currentRun.reporting.featureWarning(fixPosition(pos), featureName, featureDesc, featureTrait, construct, required)
 
 
     // nextOuter determines which context is searched next for implicits
@@ -1239,7 +1243,7 @@ trait Contexts { self: Analyzer =>
     type Error = AbsTypeError
     type Warning = (Position, String)
 
-    def issue(err: AbsTypeError)(implicit context: Context): Unit = handleError(err.errPos, addDiagString(err.errMsg))
+    def issue(err: AbsTypeError)(implicit context: Context): Unit = handleError(context.fixPosition(err.errPos), addDiagString(err.errMsg))
 
     protected def handleError(pos: Position, msg: String): Unit
     protected def handleSuppressedAmbiguous(err: AbsAmbiguousTypeError): Unit = ()
@@ -1256,7 +1260,7 @@ trait Contexts { self: Analyzer =>
      *  - else, let this context reporter decide
      */
     final def issueAmbiguousError(err: AbsAmbiguousTypeError)(implicit context: Context): Unit =
-      if (context.ambiguousErrors) reporter.error(err.errPos, addDiagString(err.errMsg)) // force reporting... see TODO above
+      if (context.ambiguousErrors) reporter.error(context.fixPosition(err.errPos), addDiagString(err.errMsg)) // force reporting... see TODO above
       else handleSuppressedAmbiguous(err)
 
     @inline final def withFreshErrorBuffer[T](expr: => T): T = {