[Foundation] searching in slices should index relative to the slice (swiftlang#14851)

Author: phausler

stdlib/public/SDK/Foundation/Data.swift

@@ -1425,17 +1425,17 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
         let nsRange : NSRange
         if let r = range {
             _validateRange(r)
-            nsRange = NSRange(location: r.lowerBound, length: r.upperBound - r.lowerBound)
+            nsRange = NSRange(location: r.lowerBound - startIndex, length: r.upperBound - r.lowerBound)
         } else {
-            nsRange = NSRange(location: 0, length: _backing.length)
+            nsRange = NSRange(location: 0, length: count)
         }
         let result = _backing.withInteriorPointerReference(_sliceRange) {
             $0.range(of: dataToFind, options: options, in: nsRange)
         }
         if result.location == NSNotFound {
             return nil
         }
-        return result.location..<(result.location + result.length)
+        return (result.location + startIndex)..<((result.location + startIndex) + result.length)
     }
 
     /// Enumerate the contents of the data.

test/stdlib/TestData.swift

@@ -3741,6 +3741,14 @@ class TestData : TestDataSuper {
 
         expectEqual(Data(bytes: [4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5]), t)
     }
+
+    func test_rangeOfSlice() {
+        let data = "FooBar".data(using: .ascii)!
+        let slice = data[3...] // Bar
+
+        let range = slice.range(of: "a".data(using: .ascii)!)
+        expectEqual(range, Range<Data.Index>(4..<5))
+    }
 }
 
 #if !FOUNDATION_XCTEST
@@ -4058,6 +4066,7 @@ DataTests.test("test_validateMutation_slice_mutableBacking_withUnsafeMutableByte
 DataTests.test("test_validateMutation_slice_customBacking_withUnsafeMutableBytes_lengthLessThanLowerBound") { TestData().test_validateMutation_slice_customBacking_withUnsafeMutableBytes_lengthLessThanLowerBound() }
 DataTests.test("test_validateMutation_slice_customMutableBacking_withUnsafeMutableBytes_lengthLessThanLowerBound") { TestData().test_validateMutation_slice_customMutableBacking_withUnsafeMutableBytes_lengthLessThanLowerBound() }
 DataTests.test("test_byte_access_of_discontiguousData") { TestData().test_byte_access_of_discontiguousData() }
+DataTests.test("test_rangeOfSlice") { TestData().test_rangeOfSlice() }
 
 // XCTest does not have a crash detection, whereas lit does
 DataTests.test("bounding failure subdata") {