RequirementMachine: Add regression test for rdar://problem/90402519

Author: slavapestov

test/Generics/rdar90402519.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift
+
+// This needs a better diagnostic. The real problem is the 'C == G<I>'
+// requirement in P2 conflicts with the one in P1.
+
+protocol P {
+  associatedtype I
+}
+
+struct G1<I> : P {}
+struct G2<T> : P {
+  typealias I = G<T>
+}
+
+struct G<T> {}
+
+protocol P0 {
+  associatedtype C : P
+}
+
+protocol P1 : P0 where C == G1<I> {
+  associatedtype I
+}
+
+protocol P2 : P1 where C == G2<I> {}
+// expected-error@-1 {{cannot build rewrite system for protocol; concrete nesting limit exceeded}}
+// expected-note@-2 {{failed rewrite rule is [P2:I].[concrete: G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<G<[P2:I]>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>] => [P2:I]}}