Add regression test for https://bugs.swift.org/browse/SR-15920

Author: slavapestov

test/Generics/sr15826.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+
+protocol AProtocol {
+  associatedtype B: BProtocol where B.A == Self
+}
+
+protocol BProtocol {
+  associatedtype A: AProtocol
+}
+
+protocol CProtocol {
+  associatedtype A: AProtocol
+}
+
+protocol DProtocol {
+  associatedtype A: AProtocol
+
+  // CHECK-LABEL: .DProtocol.foo(c:)@
+  // CHECK-NEXT: <Self, C where Self : DProtocol, C : CProtocol, Self.[DProtocol]A == C.[CProtocol]A>
+  func foo<C: CProtocol>(c: C) where C.A == A
+}
+
+struct Foo<Value> {
+  // CHECK-LABEL: .Foo.bar(c:)@
+  // CHECK-NEXT: <Value, C where Value == C.[CProtocol]A, C : CProtocol>
+  func bar<C: CProtocol>(c: C) where C.A == Value {}
+}