don't crash on unsupported pattern -- diagnose

2 files changed, 2 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/Printers.scala b/src/compiler/scala/tools/nsc/ast/Printers.scala
index 8b92f0acd6..885fc3f518 100644
--- a/src/compiler/scala/tools/nsc/ast/Printers.scala
+++ b/src/compiler/scala/tools/nsc/ast/Printers.scala
@@ -278,6 +278,7 @@ trait Printers extends reflect.internal.Printers { this: Global =>
 
   def asString(t: Tree): String = render(t, newStandardTreePrinter, settings.printtypes.value, settings.uniqid.value, settings.Yshowsymkinds.value)
   def asCompactString(t: Tree): String = render(t, newCompactTreePrinter, settings.printtypes.value, settings.uniqid.value, settings.Yshowsymkinds.value)
+  def asCompactDebugString(t: Tree): String = render(t, newCompactTreePrinter, true, true, true)
 
   def newStandardTreePrinter(writer: PrintWriter): TreePrinter = new TreePrinter(writer)
   def newStandardTreePrinter(stream: OutputStream): TreePrinter = newStandardTreePrinter(new PrintWriter(stream))
diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
index 662de79e24..ba0e7305c0 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
@@ -521,7 +521,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
         // case Star(_) | ArrayValue  => error("stone age pattern relics encountered!")
 
         case _                       =>
-          error("unsupported pattern: "+ patTree +"(a "+ patTree.getClass +")")
+          typer.context.unit.error(patTree.pos, s"unsupported pattern: $patTree (a ${patTree.getClass}).\n This is a scalac bug. Tree diagnostics: ${asCompactDebugString(patTree)}.")
           noFurtherSubPats()
       }