[nomaster] SI-8152 Backport variance validator performance fix

	% time qbin/scalac test/files/pos/t8146-performance.scala

	real	0m2.015s
	user	0m2.892s
	sys	0m0.215s

	% time scalac-hash v2.10.3 test/files/pos/t8146-performance.scala

	real	1m13.652s
	user	1m14.245s
	sys	0m0.508s

Cherry-picks one hunk from 882f8e64.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/pos/t8152-performance.scala b/test/files/pos/t8152-performance.scala
new file mode 100644
index 0000000000..b6d2ecd823
--- /dev/null
+++ b/test/files/pos/t8152-performance.scala
@@ -0,0 +1,13 @@
+class HListBench {
+
+  class A[H, T]
+
+  type B[H, T] = A[H, T]
+
+  // was okay
+  type T1 = A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, A[Int, Nothing]]]]]]]]]]]]]]]]]]]]]]]]]]]]
+
+  // Took over a minute to validate variance in 2.10.3!
+  type T2 = B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, B[Int, Nothing]]]]]]]]]]]]]]]]]]]]]]]]]]]]
+
+}
\ No newline at end of file