Merge pull request #4456 from practicalswift/swiftc-28400-swift-nominaltypedecl-prepareextensions

[swiftc (53 vs. 5156)] Add crasher in swift::constraints::ConstraintSystem::assignFixedType(...)

Author: CodaFi

validation-test/compiler_crashers/28400-swift-nominaltypedecl-prepareextensions.swift

@@ -0,0 +1,15 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -parse
+// REQUIRES: asserts
+// Discovered by https://github.com/airspeedswift (airspeedswift)
+
+let s1: Set<Int>? = []
+let s2: Set<Int>? = []
+let s3: Set<Int>? = []
+let x = s1 ?? s2 ?? s3 ?? []