[stdlib] Fix array slice self-assignment bug (#10958) (#11022)

Fix bug where assigning from self wasn't checking bounds of the assigned slice matched

Author: airspeedswift

stdlib/public/core/Arrays.swift.gyb

@@ -742,7 +742,10 @@ public struct ${Self}<Element>
     set(rhs) {
       _checkIndex(bounds.lowerBound)
       _checkIndex(bounds.upperBound)
-      if self[bounds]._buffer.identity != rhs._buffer.identity {
+      // If the replacement buffer has same identity, and the ranges match,
+      // then this was a pinned in-place modification, nothing further needed.
+      if self[bounds]._buffer.identity != rhs._buffer.identity
+      || bounds != rhs.startIndex..<rhs.endIndex {
         self.replaceSubrange(bounds, with: rhs)
       }
     }

test/stdlib/Inputs/CommonArrayTests.gyb

@@ -373,6 +373,23 @@ ${Suite}.test(
 // Special cases and one-off tests.
 //===----------------------------------------------------------------------===//
 
+${Suite}.test(
+  "${ArrayType}/subscriptSelfAssignDifferentRanges") {
+  var arr: ${ArrayType}<Int> = [ 1010, 1020, 1030 ]
+  arr[0..<0] = arr[...]
+  expectEqual([ 1010, 1020, 1030, 1010, 1020, 1030 ], arr)
+  arr[1..<1] = arr[1..<2]
+  expectEqual([ 1010, 1020, 1020, 1030, 1010, 1020, 1030 ], arr)
+  arr[1..<1] = arr[1..<2]
+  expectEqual([ 1010, 1020, 1020, 1020, 1030, 1010, 1020, 1030 ], arr)
+  arr[1..<2] = arr[0..<1]
+  expectEqual([ 1010, 1010, 1020, 1020, 1030, 1010, 1020, 1030 ], arr)
+  arr[0..<5] = arr.dropFirst(5)
+  expectEqual([ 1010, 1020, 1030, 1010, 1020, 1030 ], arr)
+  arr[...] = arr[0..<0]
+  expectEqual([  ], arr)
+}
+
 ${Suite}.test("${ArrayType}<Void>/map") {
   // This code used to crash because it generated an array of Void with
   // stride == 0.