don't check exhaustivity involving user-defined unapplySeq

Author: Adriaan Moors

src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala

@@ -1973,6 +1973,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
               case Some(0) if testedBinder.tpe.typeSymbol == ListClass => // extractor.symbol.owner == SeqFactory
                 EqualityCond(binderToUniqueTree(p.prevBinder), unique(Ident(NilModule) setType NilModule.tpe))
               case _ =>
+                backoff = true
                 super.treeMakerToCond(tm)
             }
           case ExtractorTreeMaker(_, _, _, _) =>

test/files/neg/patmatexhaust.check

@@ -18,10 +18,6 @@ patmatexhaust.scala:53: error: match may not be exhaustive.
 It would fail on the following input: Gp()
     def ma5(x:Deep) = x match {
                       ^
-patmatexhaust.scala:59: error: match may not be exhaustive.
-It would fail on the following input: Nil
-  def ma6()  = List(1,2) match { // give up
-                   ^
 patmatexhaust.scala:75: error: match may not be exhaustive.
 It would fail on the following input: B()
   def ma9(x: B) = x match {
@@ -38,4 +34,4 @@ patmatexhaust.scala:126: error: match may not be exhaustive.
 It would fail on the following input: C1()
     def ma10(x: C) = x match { // not exhaustive: C1 is not abstract.
                      ^
-10 errors found
+9 errors found

test/files/pos/exhaustive_heuristics.scala

@@ -12,5 +12,15 @@ object Test {
     case _ if turnOffChecks =>
   }
 
-  // TODO: we back off when there are any user-defined extractors
+  // we back off when there are any user-defined extractors
+  // in fact this is exhaustive, but we pretend we don't know since List's unapplySeq is not special to the compiler
+  // to compensate our ignorance, we back off
+  // well, in truth, we do rewrite List() to Nil, but otherwise we do nothing
+  // the full rewrite List(a, b) to a :: b :: Nil, for example is planned (but not sure it's a good idea)
+  List(true, false) match {
+    case List(_, _, _*) =>
+    case List(node, _*)  =>
+    case Nil =>
+  }
+
 }