Fix Substring.removeSubrange for entire substring (#60744)

fix start and end

fix test

Author: Azoy

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -483,9 +483,13 @@ extension _StringGuts {
       let newUTF8Count =
         oldUTF8Count + newUTF8Subrange.count - oldUTF8SubrangeCount
 
-      // Get the character stride in the entire string, not just the substring.
-      // (Characters in a substring may end beyond the bounds of it.)
-      let newStride = _opaqueCharacterStride(startingAt: utf8StartOffset)
+      var newStride = 0
+
+      if !newUTF8Subrange.isEmpty {
+        // Get the character stride in the entire string, not just the substring.
+        // (Characters in a substring may end beyond the bounds of it.)
+        newStride = _opaqueCharacterStride(startingAt: utf8StartOffset)
+      }
 
       startIndex = String.Index(
         encodedOffset: utf8StartOffset,
@@ -522,9 +526,14 @@ extension _StringGuts {
     // needs to look ahead by more than one Unicode scalar.)
     let oldStride = startIndex.characterStride ?? 0
     if oldRange.lowerBound <= oldBounds.lowerBound &+ oldStride {
-      // Get the character stride in the entire string, not just the substring.
-      // (Characters in a substring may end beyond the bounds of it.)
-      let newStride = _opaqueCharacterStride(startingAt: newBounds.lowerBound)
+      var newStride = 0
+
+      if !newBounds.isEmpty {
+        // Get the character stride in the entire string, not just the substring.
+        // (Characters in a substring may end beyond the bounds of it.)
+        newStride = _opaqueCharacterStride(startingAt: newBounds.lowerBound)
+      }
+
       var newStart = String.Index(
         encodedOffset: newBounds.lowerBound,
         characterStride: newStride

test/stdlib/StringIndex.swift

@@ -1059,3 +1059,28 @@ suite.test("String.replaceSubrange index validation")
     }
   }
 }
+
+suite.test("Substring.removeSubrange entire range") {
+  guard #available(SwiftStdlib 5.7, *) else {
+    // This is a regression found in 5.7+
+    return
+  }
+
+  var a: Substring = "abcdef"
+  let aStart = a.startIndex
+  let aEnd = a.endIndex
+
+  a.removeSubrange(aStart ..< aEnd)
+
+  expectTrue(a.isEmpty)
+
+#if _runtime(_ObjC)
+  var b: Substring = ("å∫ç∂éƒ" as NSString) as Substring
+  let bStart = b.startIndex
+  let bEnd = b.endIndex
+
+  b.removeSubrange(bStart ..< bEnd)
+
+  expectTrue(b.isEmpty)
+#endif
+}