[test] Add test case for rdar://126583098

Author: hamishknight

test/Constraints/rdar126583098.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-emit-silgen %s -verify
+
+protocol P {
+  var d: Double? { get }
+}
+
+// rdar://126583098 - Make sure this compiles.
+func foo<each S: Sequence>(_ xss: (repeat (each S))) where repeat (each S).Element: P {
+  var n: Double = 0
+  for xs in repeat each xss {
+    for x in xs {
+      n += x.d ?? 0
+    }
+  }
+}