[NFC] Add a test case for user-defined tuple conformances.

Author: hborla

test/Generics/tuple-conformances.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-variadic-generics -parse-stdlib
+
+import Swift
+
+protocol P {
+  associatedtype A
+  associatedtype B
+
+  func f()
+}
+
+extension Builtin.TheTupleType: P where Elements: P {
+  typealias A = (Elements.A...)
+  typealias B = Float
+  func f() {}
+}
+
+extension Int: P {
+  typealias A = Int
+  typealias B = String
+  func f() {}
+}
+
+func returnsPA<T: P>(_: T) -> T.A.Type {}
+func returnsPB<T: P>(_: T) -> T.B.Type {}
+
+func same<T>(_: T, _: T) {}
+
+func useConformance() {
+  same(returnsPA((1, 2, 3)), (Int, Int, Int).self)
+  same(returnsPB((1, 2, 3)), Float.self)
+
+  (1, 2, 3).f()
+}