Merge pull request #8003 from practicalswift/swiftc-28716-unreachable-executed-at-swift-lib-ast-type-cpp-1215

[swiftc (69 vs. 5458)] Add crasher in swift::SyntaxSugarType::getImplementationType(...)

Author: practicalswift

validation-test/compiler_crashers/28716-unreachable-executed-at-swift-lib-ast-type-cpp-1215.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: OS=linux-gnu
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+func o(UInt=_=1 + 1 t){a