SI-7459 Handle pattern binders used as prefixes in TypeTrees.

Match translation was incorrect for:

    case t => new t.C
    case D(t) => new d.C

We would end up with Types in TypeTrees referring to the wrong
symbols, e.g:

    // t7459a.scala
    ((x0$1: this.LM) => {
      case <synthetic> val x1: this.LM = x0$1;
      case4(){
       matchEnd3(new tttt.Node[Any]())
      };
      matchEnd3(x: Any){
        x
    }

Or:
    // t7459b.scala
     ((x0$1: CC) => {
       case <synthetic> val x1: CC = x0$1;
       case4(){
         if (x1.ne(null))
           matchEnd3(new tttt.Node[Any]())
         else
           case5()
       };

This commit:

  - Changes `bindSubPats` to traverse types, as well as terms,
    in search of references to bound symbols
  - Changes `Substitution` to reuse `Tree#substituteSymbols` rather
    than the home-brew substitution from `Tree`s to `Tree`s, if the
    `to` trees are all `Ident`s
  - extends `substIdentsForTrees` to substitute selections like
    `x1.caseField` into types.

I had to dance around the awkward handling of "swatches" (exception
handlers that can be implemented with JVM native type switches) by
duplicating trees to avoid seeing the results of `substituteSymbols`
in `caseDefs` after we abandon that approach if we detect the
patterns are too complex late in the game.

I also had to add an escape hatch for the "type selection from
volatile type" check in the type checker. Without this, the
translation of `pos/t7459c.scala`:

    case <synthetic> val x1: _$1 = (null: Test.Mirror[_]).universe;
    case5(){
      if (x1.isInstanceOf[Test.JavaUniverse])
        {
          <synthetic> val x2: _$1 with Test.JavaUniverse = (x1.asInstanceOf[_$1 with Test.JavaUniverse]: _$1 with Test.JavaUniverse);
          matchEnd4({
            val ju1: Test.JavaUniverse = x2;
            val f: () => x2.Type = (() => (null: x2.TypeTag[Nothing]).tpe);

.. triggers that error at `x2.TypeTag`.

5 files changed, 53 insertions, 13 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
index 21e90b1d78..50fa38e44d 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
@@ -266,7 +266,7 @@ trait MatchApproximation extends TreeAndTypeAnalysis with ScalaLogic with MatchT
       // the type of the binder passed to the first invocation
       // determines the type of the tree that'll be returned for that binder as of then
       final def binderToUniqueTree(b: Symbol) =
-        unique(accumSubst(normalize(CODE.REF(b))), b.tpe)
+        unique(accumSubst(normalize(gen.mkAttributedStableRef(b))), b.tpe)
 
       // note that the sequencing of operations is important: must visit in same order as match execution
       // binderToUniqueTree uses the type of the first symbol that was encountered as the type for all future binders
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
index d862805a07..27360fea53 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
@@ -248,7 +248,10 @@ trait MatchTranslation {
       if (caseDefs forall treeInfo.isCatchCase) caseDefs
       else {
         val swatches = { // switch-catches
-          val bindersAndCases = caseDefs map { caseDef =>
+          // SI-7459 must duplicate here as we haven't commited to switch emission, and just figuring out
+          //         if we can ends up mutating `caseDefs` down in the use of `substituteSymbols` in
+          //         `TypedSubstitution#Substitution`. That is called indirectly by `emitTypeSwitch`.
+          val bindersAndCases = caseDefs.map(_.duplicate) map { caseDef =>
             // generate a fresh symbol for each case, hoping we'll end up emitting a type-switch (we don't have a global scrut there)
             // if we fail to emit a fine-grained switch, have to do translateCase again with a single scrutSym (TODO: uniformize substitution on treemakers so we can avoid this)
             val caseScrutSym = freshSym(pos, pureType(ThrowableTpe))
@@ -518,7 +521,7 @@ trait MatchTranslation {
       // reference the (i-1)th case accessor if it exists, otherwise the (i-1)th tuple component
       override protected def tupleSel(binder: Symbol)(i: Int): Tree = {
         val accessors = binder.caseFieldAccessors
-        if (accessors isDefinedAt (i-1)) REF(binder) DOT accessors(i-1)
+        if (accessors isDefinedAt (i-1)) gen.mkAttributedStableRef(binder) DOT accessors(i-1)
         else codegen.tupleSel(binder)(i) // this won't type check for case classes, as they do not inherit ProductN
       }
     }
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
index 3abec521df..fb63ca7dc3 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
@@ -166,8 +166,17 @@ trait MatchTreeMaking extends MatchCodeGen with Debugging {
           val usedBinders = new mutable.HashSet[Symbol]()
           // all potentially stored subpat binders
           val potentiallyStoredBinders = stored.unzip._1.toSet
+          def ref(sym: Symbol) =
+            if (potentiallyStoredBinders(sym)) usedBinders += sym
           // compute intersection of all symbols in the tree `in` and all potentially stored subpat binders
-          in.foreach(t => if (potentiallyStoredBinders(t.symbol)) usedBinders += t.symbol)
+          in.foreach {
+            case tt: TypeTree =>
+              tt.tpe foreach { // SI-7459 e.g. case Prod(t) => new t.u.Foo
+                case SingleType(_, sym) => ref(sym)
+                case _ =>
+              }
+            case t => ref(t.symbol)
+          }
 
           if (usedBinders.isEmpty) in
           else {
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/PatternMatching.scala b/src/compiler/scala/tools/nsc/transform/patmat/PatternMatching.scala
index ef50e083a1..d35aad964d 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/PatternMatching.scala
@@ -12,7 +12,7 @@ import scala.language.postfixOps
 import scala.tools.nsc.transform.TypingTransformers
 import scala.tools.nsc.transform.Transform
 import scala.reflect.internal.util.Statistics
-import scala.reflect.internal.Types
+import scala.reflect.internal.{Mode, Types}
 import scala.reflect.internal.util.Position
 
 /** Translate pattern matching.
@@ -198,33 +198,57 @@ trait Interface extends ast.TreeDSL {
     }
 
     class Substitution(val from: List[Symbol], val to: List[Tree]) {
-      import global.{Transformer, Ident, NoType}
+      import global.{Transformer, Ident, NoType, TypeTree, SingleType}
 
       // We must explicitly type the trees that we replace inside some other tree, since the latter may already have been typed,
       // and will thus not be retyped. This means we might end up with untyped subtrees inside bigger, typed trees.
       def apply(tree: Tree): Tree = {
         // according to -Ystatistics 10% of translateMatch's time is spent in this method...
         // since about half of the typedSubst's end up being no-ops, the check below shaves off 5% of the time spent in typedSubst
-        if (!tree.exists { case i@Ident(_) => from contains i.symbol case _ => false}) tree
-        else (new Transformer {
+        val toIdents = to.forall(_.isInstanceOf[Ident])
+        val containsSym = tree.exists {
+          case i@Ident(_) => from contains i.symbol
+          case tt: TypeTree => tt.tpe.exists {
+            case SingleType(_, sym) =>
+              (from contains sym) && {
+                if (!toIdents) global.devWarning(s"Unexpected substitution of non-Ident into TypeTree `$tt`, subst= $this")
+                true
+              }
+            case _ => false
+          }
+          case _          => false
+        }
+        val toSyms = to.map(_.symbol)
+        object substIdentsForTrees extends Transformer {
           private def typedIfOrigTyped(to: Tree, origTp: Type): Tree =
             if (origTp == null || origTp == NoType) to
             // important: only type when actually substing and when original tree was typed
             // (don't need to use origTp as the expected type, though, and can't always do this anyway due to unknown type params stemming from polymorphic extractors)
             else typer.typed(to)
 
+          def typedStable(t: Tree) = typer.typed(t.shallowDuplicate, Mode.MonoQualifierModes | Mode.TYPEPATmode)
+          lazy val toTypes: List[Type] = to map (tree => typedStable(tree).tpe)
+
           override def transform(tree: Tree): Tree = {
             def subst(from: List[Symbol], to: List[Tree]): Tree =
               if (from.isEmpty) tree
-              else if (tree.symbol == from.head) typedIfOrigTyped(to.head.shallowDuplicate.setPos(tree.pos), tree.tpe)
+              else if (tree.symbol == from.head) typedIfOrigTyped(typedStable(to.head).setPos(tree.pos), tree.tpe)
               else subst(from.tail, to.tail)
 
-            tree match {
+            val tree1 = tree match {
               case Ident(_) => subst(from, to)
               case _        => super.transform(tree)
             }
+            tree1.modifyType(_.substituteTypes(from, toTypes))
           }
-        }).transform(tree)
+        }
+        if (containsSym) {
+          if (to.forall(_.isInstanceOf[Ident]))
+            tree.duplicate.substituteSymbols(from, to.map(_.symbol)) // SI-7459 catches `case t => new t.Foo`
+          else
+            substIdentsForTrees.transform(tree)
+        }
+        else tree
       }
 
 
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 70acb03584..f93f8a781d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3727,8 +3727,12 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           case TypeRef(pre, sym, args) =>
             if (sym.isAliasType && containsLocal(tp) && (tp.dealias ne tp)) apply(tp.dealias)
             else {
-              if (pre.isVolatile)
-                InferTypeWithVolatileTypeSelectionError(tree, pre)
+              if (pre.isVolatile) pre match {
+                case SingleType(_, sym) if sym.isSynthetic && isPastTyper =>
+                  debuglog(s"ignoring volatility of prefix in pattern matcher generated inferred type: $tp") // See pos/t7459c.scala
+                case _ =>
+                  InferTypeWithVolatileTypeSelectionError(tree, pre)
+              }
               mapOver(tp)
             }
           case _ =>