[String] String.Index.init(_:within:) bounds checks

Bounds check the given index for String.Index's generic initializer
that makes sure a passed index is a valid one for the given
StringProtocol.

Author: milseman

stdlib/public/core/StringIndexConversions.swift

@@ -12,12 +12,15 @@
 
 extension String.Index {
   private init?<S: StringProtocol>(
-    _ sourcePosition: String.Index, _genericWithin target: S
+    _ idx: String.Index, _genericWithin target: S
   ) {
-    guard target._wholeGuts.isOnGraphemeClusterBoundary(sourcePosition) else {
+    guard target._wholeGuts.isOnGraphemeClusterBoundary(idx),
+          idx >= target.startIndex && idx <= target.endIndex
+    else {
       return nil
     }
-    self = sourcePosition
+
+    self = idx
   }
 
   /// Creates an index in the given string that corresponds exactly to the

test/stdlib/StringIndex.swift

@@ -215,6 +215,11 @@ StringIndexTests.test("String.Index(_:within) / Range<String.Index>(_:in:)") {
       String.Index(idx, within: str), String.Index(idx, within: substr))
   }
 
+  expectNil(String.Index(str.startIndex, within: str.dropFirst()))
+  expectNil(String.Index(str.endIndex, within: str.dropLast()))
+  expectNotNil(String.Index(str.startIndex, within: str))
+  expectNotNil(String.Index(str.endIndex, within: str))
+
   let utf16Count = str.utf16.count
   let utf16Indices = Array(str.utf16.indices) + [str.utf16.endIndex]
   for location in 0..<utf16Count {