[Test] Add test case for now-fixed rdar://problem/39805133

test/multifile/Inputs/protocol-conformance-rdar39805133-other.swift

@@ -0,0 +1 @@
+protocol DefaultInit { init() }

test/multifile/protocol-conformance-rdar39805133.swift

@@ -0,0 +1,29 @@
+// RUN: %target-swift-frontend -emit-ir -o - -primary-file %s %S/Inputs/protocol-conformance-rdar39805133-other.swift -module-name foo
+// RUN: %target-swift-frontend -emit-ir -o - %s -primary-file %S/Inputs/protocol-conformance-rdar39805133-other.swift -module-name foo
+
+protocol _Int : DefaultInit {
+    associatedtype Minus1 : _Int
+    associatedtype Plus1 : _Int = Inc<Self>
+    static var value: Int { get }
+}
+
+struct Inc<T : _Int> : _Int {
+    typealias Minus1 = T
+    static var value: Int { return T.value + 1 }
+}
+
+extension _Int {
+    var plus1: Plus1 { return Plus1() }
+    var minus1: Minus1 { return Minus1() }
+}
+
+struct _0_ : _Int {
+    typealias Minus1 = _0_//Underflow
+    static let value = 0
+}
+
+let _0 = _0_()
+protocol AtLeast0 : _Int {}
+extension _0_ : AtLeast0 {}
+protocol AtLeast1 : AtLeast0 {}
+extension Inc : AtLeast1, AtLeast0 where T == _0_ {}