[Foundation] IndexPath should not cause numeric overflow in addition to calculate hash values (swiftlang#9476)

Author: phausler

stdlib/public/SDK/Foundation/IndexPath.swift

@@ -658,7 +658,7 @@ public struct IndexPath : ReferenceConvertible, Equatable, Hashable, MutableColl
             let totalBits = MemoryLayout<Int>.size * 8
             let lengthBits = 8
             let firstIndexBits = (totalBits - lengthBits) / 2
-            return count + (first << lengthBits) + (last << (lengthBits + firstIndexBits))
+            return count &+ (first << lengthBits) &+ (last << (lengthBits + firstIndexBits))
         }
 
         switch _indexes {

test/stdlib/TestIndexPath.swift

@@ -226,6 +226,8 @@ class TestIndexPath: TestIndexPathSuper {
         let ip2: IndexPath = [1, 1, 1]
 
         expectNotEqual(ip1.hashValue, ip2.hashValue)
+
+        IndexPath(indexes: [Int.max >> 8, 2, Int.max >> 36]).hashValue // this should not cause an overflow crash
     }
 
     func testEquality() {