Cleaning up type alias usage.

I determined that many if not most of the calls to .normalize
have no intent beyond dealiasing the type. In light of this I
went call site to call site knocking on doors and asking why
exactly they were calling any of

  .normalize
  .widen.normalize
  .normalize.widen

and if I didn't like their answers they found themselves
introduced to 'dropAliasesAndSingleTypes', the recursive widener
and dealiaser which I concluded is necessary after all.

Discovered that the object called 'deAlias' actually depends
upon calling 'normalize', not 'dealias'. Decided this was
sufficient cause to rename it to 'normalizeAliases'.

Created dealiasWiden and dealiasWidenChain.

Dropped dropAliasesAndSingleTypes in favor of methods
on Type alongside dealias and widen (Type#dealiasWiden).
These should reduce the number of "hey, the type alias doesn't work" bugs.

1 files changed, 14 insertions, 17 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 8d6e0c3b85..ed1e6d01e8 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -439,8 +439,8 @@ trait Implicits {
       val start = if (Statistics.canEnable) Statistics.startTimer(matchesPtNanos) else null
       val result = normSubType(tp, pt) || isView && {
         pt match {
-          case TypeRef(_, Function1.Sym, args) =>
-            matchesPtView(tp, args.head, args.tail.head, undet)
+          case TypeRef(_, Function1.Sym, arg1 :: arg2 :: Nil) =>
+            matchesPtView(tp, arg1, arg2, undet)
           case _ =>
             false
         }
@@ -484,7 +484,7 @@ trait Implicits {
             loop(restpe, pt)
           else pt match {
             case tr @ TypeRef(pre, sym, args) =>
-              if (sym.isAliasType) loop(tp, pt.normalize)
+              if (sym.isAliasType) loop(tp, pt.dealias)
               else if (sym.isAbstractType) loop(tp, pt.bounds.lo)
               else {
                 val len = args.length - 1
@@ -528,18 +528,15 @@ trait Implicits {
      *  to a final true or false.
      */
     private def isPlausiblySubType(tp1: Type, tp2: Type) = !isImpossibleSubType(tp1, tp2)
-    private def isImpossibleSubType(tp1: Type, tp2: Type) = tp1.normalize.widen match {
-      case tr1 @ TypeRef(_, sym1, _) =>
-        // We can only rule out a subtype relationship if the left hand
-        // side is a class, else we may not know enough.
-        sym1.isClass && (tp2.normalize.widen match {
-          case TypeRef(_, sym2, _) =>
-             sym2.isClass && !(sym1 isWeakSubClass sym2)
-          case RefinedType(parents, decls) =>
-            decls.nonEmpty &&
-            tr1.member(decls.head.name) == NoSymbol
-          case _ => false
-        })
+    private def isImpossibleSubType(tp1: Type, tp2: Type) = tp1.dealiasWiden match {
+      // We can only rule out a subtype relationship if the left hand
+      // side is a class, else we may not know enough.
+      case tr1 @ TypeRef(_, sym1, _) if sym1.isClass =>
+        tp2.dealiasWiden match {
+          case TypeRef(_, sym2, _)         => sym2.isClass && !(sym1 isWeakSubClass sym2)
+          case RefinedType(parents, decls) => decls.nonEmpty && tr1.member(decls.head.name) == NoSymbol
+          case _                           => false
+        }
       case _ => false
     }
 
@@ -1010,7 +1007,7 @@ trait Implicits {
                 args foreach (getParts(_))
               }
             } else if (sym.isAliasType) {
-              getParts(tp.normalize)
+              getParts(tp.dealias)
             } else if (sym.isAbstractType) {
               getParts(tp.bounds.hi)
             }
@@ -1168,7 +1165,7 @@ trait Implicits {
         implicit def wrapResult(tree: Tree): SearchResult =
           if (tree == EmptyTree) SearchFailure else new SearchResult(tree, if (from.isEmpty) EmptyTreeTypeSubstituter else new TreeTypeSubstituter(from, to))
 
-        val tp1 = tp0.normalize
+        val tp1 = tp0.dealias
         tp1 match {
           case ThisType(_) | SingleType(_, _) =>
             // can't generate a reference to a value that's abstracted over by an existential