SI-6891 Fix value class + tailrec crasher.

rhs.substituteSymbols(old, new) leaves us with:

    def loop#12225(x#12226: A#15491): scala#21.this.Unit#1615 =
      loop#12225(x#12226)

In which the TermSymbol x#12226 has a stale info, pointing at
the A#7274, the class type parameter, rather than A#15491,
the corresponding type parameter of the synthetic backing
method.

I've improved `TreeSymSubstituter` to substitute not
only `Tree#{tpe, symbol}`, but also `DefTree#sym.info`.

The `pos` test that triggered the new code path are
listed here: https://gist.github.com/4575687

AFAICS, no special treatment of Function, Return, or Import
is needed in TreeSymSubstutor.

1 files changed, 26 insertions, 0 deletions

diff --git a/test/files/pos/t6891.scala b/test/files/pos/t6891.scala
new file mode 100644
index 0000000000..bed2d0d777
--- /dev/null
+++ b/test/files/pos/t6891.scala
@@ -0,0 +1,26 @@
+object O {
+  implicit class Foo[A](val value: String) extends AnyVal {
+    def bippy() = {
+      @annotation.tailrec def loop(x: A): Unit = loop(x)
+      ()
+    }
+
+    def boppy() = {
+      @annotation.tailrec def loop(x: value.type): Unit = loop(x)
+      ()
+    }
+
+    def beppy[C](c: => C) = {
+      () => c
+      @annotation.tailrec def loop(x: value.type): Unit = loop(x)
+        () => c
+      ()
+    }
+  }
+  // uncaught exception during compilation: Types$TypeError("type mismatch;
+  //  found   : A(in method bippy$extension)
+  //  required: A(in class Foo)") @ scala.tools.nsc.typechecker.Contexts$Context.issueCommon(Contexts.scala:396)
+  // error: scala.reflect.internal.Types$TypeError: type mismatch;
+  //  found   : A(in method bippy$extension)
+  //  required: A(in class Foo)
+}