No longer crash on NoSymbol.owner.

Historically calling NoSymbol.owner has crashed the compiler.
With this commit, NoSymbol owns itself. This is consistent with
the way ownership chains are handled elsewhere in the compiler
(e.g. NoContext.owner is NoContext, NoSymbol.enclClass is
NoSymbol, and so on) and frees every call site which handles
symbols from having to perform precondition tests against
NoSymbol.

Since calling NoSymbol.owner sometimes (not always) indicates
a bug which we'd like to catch sooner than later, I have
introduced a couple more methods for selected call sites.

  def owner: Symbol       // NoSymbol.owner is self, log if -Xdev
  def safeOwner: Symbol   // NoSymbol.owner is self, ignore
  def assertOwner: Symbol // NoSymbol.owner is fatal

The idea is that everyone can call sym.owner without undue anxiety
or paranoid null-like tests. When compiling under -Xdev calls to
`owner` are logged with a stack trace, so any call sites for which
that is an expected occurrence should call safeOwner instead to
communicate the intention and stay out of the log. Conversely, any
call site where crashing on the owner call was a desirable behavior
can opt into calling assertOwner.

This commit also includes all the safeOwner calls necessary to
give us a silent log when compiling scala.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
index f089c8f5a5..dfd18d089e 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
@@ -222,7 +222,7 @@ trait MatchApproximation extends TreeAndTypeAnalysis with ScalaLogic with MatchT
           // so that we don't introduce new aliases for existing symbols, thus keeping the set of bound symbols minimal
           val (boundSubst, unboundSubst) = (subst.from zip subst.to) partition {
             case (f, t) =>
-              t.isInstanceOf[Ident] && (t.symbol ne NoSymbol) && pointsToBound(f)
+              t.isInstanceOf[Ident] && t.symbol.exists && pointsToBound(f)
           }
           val (boundFrom, boundTo) = boundSubst.unzip
           val (unboundFrom, unboundTo) = unboundSubst.unzip
@@ -624,9 +624,9 @@ trait MatchAnalysis extends MatchApproximation {
         private lazy val uniqueEqualTo = equalTo filterNot (subsumed => equalTo.exists(better => (better ne subsumed) && instanceOfTpImplies(better.tp, subsumed.tp)))
         private lazy val prunedEqualTo = uniqueEqualTo filterNot (subsumed => variable.staticTpCheckable <:< subsumed.tp)
         private lazy val ctor       = (prunedEqualTo match { case List(TypeConst(tp)) => tp case _ => variable.staticTpCheckable }).typeSymbol.primaryConstructor
-        private lazy val ctorParams = if (ctor == NoSymbol || ctor.paramss.isEmpty) Nil else ctor.paramss.head
-        private lazy val cls        = if (ctor == NoSymbol) NoSymbol else ctor.owner
-        private lazy val caseFieldAccs = if (cls == NoSymbol) Nil else cls.caseFieldAccessors
+        private lazy val ctorParams = if (ctor.paramss.isEmpty) Nil else ctor.paramss.head
+        private lazy val cls        = ctor.safeOwner
+        private lazy val caseFieldAccs = cls.caseFieldAccessors
 
         def addField(symbol: Symbol, assign: VariableAssignment) {
           // SI-7669 Only register this field if if this class contains it.