Merge pull request #9386 from practicalswift/swiftc-28748-genericenv-nullptr-too-much-circularity

[swiftc (34 vs. 5523)] Add crasher in swift::AbstractTypeParamDecl::getSuperclass(...)

Author: practicalswift

validation-test/compiler_crashers/28748-genericenv-nullptr-too-much-circularity.swift

@@ -0,0 +1,10 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// REQUIRES: asserts
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+protocol A:a{{}typealias e:a}{}class a:A{typealias e:b