SI-9157 Avoid exponential blowup with chained type projections

Calling `findMember` in the enclosed test was calling into
`NonClassTypeRef#relativeInfo` an exponentially-increasing number
of times, with respect to the length of the chained type projections.

The numbers of calls increased as: 26, 326, 3336, 33446, 334556.
Can any pattern spotters in the crowd that can identify the sequence?
(I can't.)

Tracing the calls saw we were computing the same `memberType`
repeatedly. This part of the method was not guarded by the cache.

I have changed the method to use the standard idiom of using the
current period for cache invalidation. The enclosed test now compiles
promptly, rather than in geological time.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/pos/t9157.scala b/test/files/pos/t9157.scala
new file mode 100644
index 0000000000..e178b5d84d
--- /dev/null
+++ b/test/files/pos/t9157.scala
@@ -0,0 +1,13 @@
+trait Flow[-In, +Out] {
+  type Repr[+O] <: Flow[In, O]
+  def map: Repr[String]
+}
+
+class Test {
+  // typechecking was exponentially slow wrt the number of projections here.
+  def slowFlow(
+    f: Flow[String,String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]#Repr[String]
+  ) = {
+    f.map
+  }
+}