SI-6675 Avoid spurious warning about pattern bind arity.

In 692372ce, we added a warning (under -Xlint) when binding
a `TupleN` in to a single pattern binder, which wasn't allowed
before 2.10.0, and more often than not represents a bug.

However, that warning overstretched, and warned even when
using a Tuple Pattern to bind to the elements of such a value.

This commit checks for this case, and avoids the spurious warnings.

A new test case is added for this case to go with the existing
test for SI-6675:

    $ ./tools/partest-ack 6675
    % tests-with-matching-paths      ...  3
    % tests-with-matching-code       ...  2
    # 3 tests to run.
    test/partest --show-diff --show-log                  \
      test/files/neg/t6675-old-patmat.scala              \
      test/files/neg/t6675.scala                         \
      test/files/pos/t6675.scala                         \
      ""

    Testing individual files
    testing: [...]/files/pos/t6675.scala                                  [  OK  ]

    Testing individual files
    testing: [...]/files/neg/t6675-old-patmat.scala                       [  OK  ]
    testing: [...]/files/neg/t6675.scala                                  [  OK  ]
    All of 3 tests were successful (elapsed time: 00:00:03)

1 files changed, 3 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index d1d6feae97..64e0603851 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3480,8 +3480,8 @@ trait Typers extends Modes with Adaptations with Tags {
       else {
         val resTp     = fun1.tpe.finalResultType.normalize
         val nbSubPats = args.length
-
-        val (formals, formalsExpanded) = extractorFormalTypes(fun0.pos, resTp, nbSubPats, fun1.symbol)
+        val (formals, formalsExpanded) =
+          extractorFormalTypes(fun0.pos, resTp, nbSubPats, fun1.symbol, treeInfo.effectivePatternArity(args))
         if (formals == null) duplErrorTree(WrongNumberOfArgsError(tree, fun))
         else {
           val args1 = typedArgs(args, mode, formals, formalsExpanded)
@@ -5297,7 +5297,7 @@ trait Typers extends Modes with Adaptations with Tags {
 
       def typedUnApply(tree: UnApply) = {
         val fun1 = typed(tree.fun)
-        val tpes = formalTypes(unapplyTypeList(tree.fun.pos, tree.fun.symbol, fun1.tpe, tree.args.length), tree.args.length)
+        val tpes = formalTypes(unapplyTypeList(tree.fun.pos, tree.fun.symbol, fun1.tpe, tree.args), tree.args.length)
         val args1 = map2(tree.args, tpes)(typedPattern)
         treeCopy.UnApply(tree, fun1, args1) setType pt
       }