SI-9050 Fix crasher with value classes, recursion

From the "Substitution is hard to do" department.

In 7babdab9a, TreeSymSubstitutor was modified to mutate the info
of symbols defined in the tree, if that symbol's info referred to
one of the `from` symbols in the substitution.

It would have been more principled to create a cloned symbol
with the updated info, and add that to the substitution. But I
wasn't able implement that correctly (let alone efficiently.)

The in-place mutation of the info of a symbol led to the crasher
in this bug: a singleton type over that symbol ends up with a stale
cached value of 'underlying'. In the enclosed test case, this leads
to a type error in the `SubstituteRecursion` of the extension
methods phase.

This commit performs a cleanup job at the end of `substituteSymbols`
by invalidating the cache of any `SingleType`-s in the tree that
refer to one of the mutated symbols.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/pos/t9050.scala b/test/files/pos/t9050.scala
new file mode 100644
index 0000000000..b1ab09f901
--- /dev/null
+++ b/test/files/pos/t9050.scala
@@ -0,0 +1,13 @@
+final class Mu[F](val value: Any) extends AnyVal {
+  def cata(f: F) {
+    // crash
+    ((y: Mu[F]) => y.cata(f))
+    // crash
+    def foo(x : Mu[F]) = x.cata(f)
+
+    // // okay
+    def x: Mu[F] = ???
+    (() => x.cata(f))
+    assert(true, cata(f))
+  }
+}