RequirementMachine: Add a small test case

Author: slavapestov

test/Generics/recursive_conformances.swift

@@ -0,0 +1,61 @@
+// RUN: %target-typecheck-verify-swift -enable-requirement-machine
+
+// Make sure the requirement machine can compute a confluent
+// completion in examples where we merge two associated types
+// with the same name, both having recursive conformance
+// requirements.
+
+// Merged two cycles of length 1:
+//
+// P1 -> P1 -> P1 -> ...
+// P2 -> P2 -> P2 -> ...
+protocol P1 {
+  associatedtype T : P1
+}
+
+protocol P2 {
+  associatedtype T : P2
+}
+
+struct S<T : P1 & P2> {}
+
+// Merged a cycle of length 2 with a cycle of length 3
+//
+// P1a -> P1b -> P1a -> P1b -> P1a -> P1b -> ...
+// P2a -> P2b -> P2c -> P2a -> P2b -> P2c -> ...
+protocol P1a {
+  associatedtype T : P1b
+}
+
+protocol P1b {
+  associatedtype T : P1a
+}
+
+protocol P2a {
+  associatedtype T : P2b
+}
+
+protocol P2b {
+  associatedtype T : P2c
+}
+
+protocol P2c {
+  associatedtype T : P2a
+}
+
+struct SS<T : P1a & P2a> {}
+
+// Merged two cycles of length 1 via an inherited associated type
+protocol Base {
+  associatedtype T : Base // expected-note {{'T' declared here}}
+}
+
+// Base     -> Base     -> Base     -> ...
+// Derived1 -> Derived1 -> Derived1 -> ...
+protocol Derived1 : Base {
+  associatedtype T : Derived1 // expected-warning {{redeclaration of associated type 'T' from protocol 'Base' is better expressed as a 'where' clause on the protocol}}
+}
+
+// Base     -> Base     -> Base     -> ...
+// Derived2 -> Derived2 -> Derived2 -> ...
+protocol Derived2 : Base where T : Derived2 {}