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`.

1 files changed, 5 insertions, 2 deletions

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
       }
     }