[TypeChecker] NFC: Add test-case for SR-4664

Resolves: rdar://problem/31764828

Author: xedin

test/Constraints/sr4664.swift

@@ -0,0 +1,22 @@
+// RUN: %target-typecheck-verify-swift
+
+struct M<T> where T : Collection { // expected-note {{where 'T' = 'X.Y'}}
+  static func f(a: T, b: T) -> [E<T.Iterator.Element>] {
+  }
+}
+
+enum E<T> {}
+
+struct S {}
+
+struct X {
+  struct Y {
+    let s: [S]
+  }
+
+  let y: [Y]
+}
+
+let x = X(y: [])
+let a = M.f(a: x.y[0], b: x.y[1])
+// expected-error@-1 {{generic struct 'M' requires that 'X.Y' conform to 'Collection'}}