SI-9855 Fix regression in extractor pattern translation

In faa5ae6, I changed the pattern matchers code generator to
use stable references (`Ident`-s with the singleton type, rather
than the widened type) to the synthetic vals used to store
intermediate results ("binders").

In the case where the scrutinee matched the unapply parameter
type of some extractor pattern, but the pattern subsequently
failed, this led to an regression.

It turns out that this was due to the way that the type of
the binder was mutated to upcast to the exact type of a subsequent
pattern in `ensureConformsTo`:

   https://github.com/scala/scala/blob/953559988/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala#L165-L174

This was added in 32c57329a as a workaround for the problem caused
in t6664.scala, when the binder type was `KList with KCons`, and
the code generator wasn't able to find the case field accessors
for `KCons` in the decls.

The change to use stable references meant that this mutation was
now observed in another part of the tree, as opposed to the 2.11.8
situation, where we had used the original, sharper type of the binder
eagerly to assign to the `Ident` that referred to it. This led to
a tree:

    Assign(Ident(x3), Ident(x1).setType(x1.tpe)

Now that we instead refer generate:

    Assign(Ident(x3), Ident(x1).setType(stableTypeFor(x1))

and we don't typecheck this until after the mutation of `x1.symbol.info`,
we can get a type error.

This commit removes this mutation of the binder type altogether, and
instead uses `aligner.wholeType`, which is based on the result type of
the `Apply(TypeTree(MethodType(params, resultType))` that encodes a
typechecked constructor pattern. In `t6624.scala`, this is `KCons`,
the case class that has the extractors as its decls.

4 files changed, 32 insertions, 17 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
index e12b8548a8..5750f8f7e7 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTranslation.scala
@@ -125,7 +125,7 @@ trait MatchTranslation {
         // TODO: paramType may contain unbound type params (run/t2800, run/t3530)
         val makers = (
           // Statically conforms to paramType
-          if (this ensureConformsTo paramType) treeMaker(binder, false, pos) :: Nil
+          if (tpe <:< paramType) treeMaker(binder, false, pos) :: Nil
           else typeTest :: extraction :: Nil
         )
         step(makers: _*)(extractor.subBoundTrees: _*)
@@ -162,16 +162,6 @@ trait MatchTranslation {
         setVarInfo(binder, paramType)
         true
       }
-      // If <:< but not =:=, no type test needed, but the tree maker relies on the binder having
-      // exactly paramType (and not just some type compatible with it.) SI-6624 shows this is necessary
-      // because apparently patBinder may have an unfortunate type (.decls don't have the case field
-      // accessors) TODO: get to the bottom of this -- I assume it happens when type checking
-      // infers a weird type for an unapply call. By going back to the parameterType for the
-      // extractor call we get a saner type, so let's just do that for now.
-      def ensureConformsTo(paramType: Type): Boolean = (
-           (tpe =:= paramType)
-        || (tpe <:< paramType) && setInfo(paramType)
-      )
 
       private def concreteType = tpe.bounds.hi
       private def unbound = unbind(tree)
@@ -396,7 +386,6 @@ trait MatchTranslation {
 
       /** Create the TreeMaker that embodies this extractor call
        *
-       * `binder` has been casted to `paramType` if necessary
        * `binderKnownNonNull` indicates whether the cast implies `binder` cannot be null
        * when `binderKnownNonNull` is `true`, `ProductExtractorTreeMaker` does not do a (redundant) null check on binder
        */
@@ -502,7 +491,7 @@ trait MatchTranslation {
        * when `binderKnownNonNull` is `true`, `ProductExtractorTreeMaker` does not do a (redundant) null check on binder
        */
       def treeMaker(binder: Symbol, binderKnownNonNull: Boolean, pos: Position): TreeMaker = {
-        val paramAccessors = binder.constrParamAccessors
+        val paramAccessors = aligner.wholeType.typeSymbol.constrParamAccessors
         val numParams = paramAccessors.length
         def paramAccessorAt(subPatIndex: Int) = paramAccessors(math.min(subPatIndex, numParams - 1))
         // binders corresponding to mutable fields should be stored (SI-5158, SI-6070)
@@ -531,7 +520,7 @@ 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
+        val accessors = aligner.wholeType.typeSymbol.caseFieldAccessors
         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/test/files/pos/t9855.scala b/test/files/pos/t9855.scala
new file mode 100644
index 0000000000..b6ac3e2432
--- /dev/null
+++ b/test/files/pos/t9855.scala
@@ -0,0 +1,10 @@
+class C {
+	def xx(verb: String, a: Array[Int]) {
+	  val reYYYY = """(\d\d\d\d)""".r
+	  verb match {
+	    case "time" if a.isEmpty =>
+	    case "time"              =>
+	    case reYYYY(y)           =>
+	  }
+	}
+}
diff --git a/test/files/pos/t9855b.scala b/test/files/pos/t9855b.scala
new file mode 100644
index 0000000000..30c58be3dc
--- /dev/null
+++ b/test/files/pos/t9855b.scala
@@ -0,0 +1,16 @@
+object Test {
+  var FALSE = false
+  def main(args: Array[String]): Unit = {
+    val SomeB = new B
+    new B() match {
+      case SomeB if FALSE =>
+      case SomeB =>
+      case Ext(_) =>
+    }
+  }
+}
+object Ext {
+  def unapply(s: A) = Some(())
+}
+class A
+class B extends A
diff --git a/test/files/run/t6288.check b/test/files/run/t6288.check
index 67877fd56d..7933f516a8 100644
--- a/test/files/run/t6288.check
+++ b/test/files/run/t6288.check
@@ -7,7 +7,7 @@
     };
     [21]def unapply([29]z: [32]<type: [32]scala.Any>): [21]Option[Int] = [56][52][52]scala.Some.apply[[52]Int]([58]-1);
     [64]{
-      [64]case <synthetic> val x1: [64]Any = [64]"";
+      [64]case <synthetic> val x1: [64]String = [64]"";
       [64]case5()[84]{
         [84]<synthetic> val o7: [84]Option[Int] = [84][84]Case3.unapply([84]x1);
         [84]if ([84]o7.isEmpty.unary_!)
@@ -30,7 +30,7 @@
     };
     [127]def unapplySeq([138]z: [141]<type: [141]scala.Any>): [127]Option[List[Int]] = [167]scala.None;
     [175]{
-      [175]case <synthetic> val x1: [175]Any = [175]"";
+      [175]case <synthetic> val x1: [175]String = [175]"";
       [175]case5()[195]{
         [195]<synthetic> val o7: [195]Option[List[Int]] = [195][195]Case4.unapplySeq([195]x1);
         [195]if ([195][195]o7.isEmpty.unary_!.&&([195][195][195][195]o7.get.!=([195]null).&&([195][195][195][195]o7.get.lengthCompare([195]1).==([195]0))))
@@ -53,7 +53,7 @@
     };
     [238]def unapply([246]z: [249]<type: [249]scala.Any>): [238]Boolean = [265]true;
     [273]{
-      [273]case <synthetic> val x1: [273]Any = [273]"";
+      [273]case <synthetic> val x1: [273]String = [273]"";
       [273]case5()[293]{
         [293]<synthetic> val o7: [293]Option[List[Int]] = [293][293]Case4.unapplySeq([293]x1);
         [293]if ([293][293]o7.isEmpty.unary_!.&&([293][293][293][293]o7.get.!=([293]null).&&([293][293][293][293]o7.get.lengthCompare([293]0).==([293]0))))