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

Author: retronym

src/reflect/scala/reflect/internal/Trees.scala

@@ -1439,6 +1439,18 @@ trait Trees extends api.Trees { self: SymbolTable =>
       if (tree.tpe ne null) tree.tpe = symSubst(tree.tpe)
       if (tree.hasSymbol) {
         subst(from, to)
+        tree match {
+          case _: DefTree =>
+            val newInfo = symSubst(tree.symbol.info)
+            if (!(newInfo =:= tree.symbol.info)) {
+              debuglog(sm"""
+                |TreeSymSubstituter: updated info of symbol ${tree.symbol}
+                |  Old: ${showRaw(tree.symbol.info, printTypes = true, printIds = true)}
+                |  New: ${showRaw(newInfo, printTypes = true, printIds = true)}""")
+              tree.symbol updateInfo newInfo
+            }
+          case _          =>
+        }
         tree match {
           case Ident(name0) if tree.symbol != NoSymbol =>
             treeCopy.Ident(tree, tree.symbol.name)

test/files/pos/t6891.scala

@@ -0,0 +1,19 @@
+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)
+      ()
+    }
+  }
+  // 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)
+}