Ill-scoped reference checking in TreeCheckers

Find trees which have an info referring to an out-of-scope
type parameter or local symbol, as could happen in the test
for SI-6981, in which tree transplanting did not substitute
symbols in symbol infos.

The enclosed, pending test for that bug that will now
fail under -Ycheck:extmethods -Xfatal-warnings.

    [Now checking: extmethods]
    [check: extmethods] The symbol, tpe or info of tree `(@scala.annotation.tailrec def loop(x: A): Unit = loop(x)) : (x: A)Unit` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: method loop, method bippy$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(val x: A = _) : A` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: value x, method loop, method bippy$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(loop(x)) : (x: A)Unit` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: method loop, method bippy$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(loop) : (x: A)Unit` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: method loop, method bippy$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(x) : A` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: value x, method loop, method bippy$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(<synthetic> val x2: O.Foo[A] = (x1.asInstanceOf[O.Foo[A]]: O.Foo[A])) : O.Foo[A]` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: value x2, method equals$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(<synthetic> val Foo$1: O.Foo[A] = x$1.asInstanceOf[O.Foo[A]]) : O.Foo[A]` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: value Foo$1, method equals$extension, object Foo, object O, package <empty>, package <root>
    [check: extmethods] The symbol, tpe or info of tree `(Foo$1) : O.Foo[A]` refers to a out-of-scope symbol, type A in class Foo. tree.symbol.ownerChain: value Foo$1, method equals$extension, object Foo, object O, package <empty>, package <root>
    error: TreeCheckers detected non-compliant trees in t6891.scala
    one error found

1 files changed, 1 insertions, 0 deletions

diff --git a/test/pending/pos/t6891.flags b/test/pending/pos/t6891.flags
new file mode 100644
index 0000000000..fe048006aa
--- /dev/null
+++ b/test/pending/pos/t6891.flags
@@ -0,0 +1 @@
+-Ycheck:extmethods -Xfatal-warnings
\ No newline at end of file