Avoid setRange with potentially incompatible types (#320)

Fixes #317

Use `sublist` on the input to get a list with the same runtime generic
as the input instead of constructing a new list with the static generic.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Adds `shuffled` to `IterableExtension`.
 - Shuffle `IterableExtension.sample` results.
+- Fix `mergeSort` when the runtime iterable generic is a subtype of the static
+  generic.
 - Require Dart `^3.1.0`
 - Mark "mixin" classes as `mixin`.
 

lib/src/algorithms.dart

@@ -232,7 +232,7 @@ void mergeSort<E>(List<E> elements,
   var middle = start + firstLength;
   var secondLength = end - middle;
   // secondLength is always the same as firstLength, or one greater.
-  var scratchSpace = List<E>.filled(secondLength, elements[start]);
+  var scratchSpace = elements.sublist(0, secondLength);
   _mergeSort(elements, identity<E>, compare, middle, end, scratchSpace, 0);
   var firstTarget = end - firstLength;
   _mergeSort(
@@ -268,7 +268,7 @@ void mergeSortBy<E, K>(List<E> elements, K Function(E element) keyOf,
   var firstLength = middle - start;
   var secondLength = end - middle;
   // secondLength is always the same as firstLength, or one greater.
-  var scratchSpace = List<E>.filled(secondLength, elements[start]);
+  var scratchSpace = elements.sublist(0, secondLength);
   _mergeSort(elements, keyOf, compare, middle, end, scratchSpace, 0);
   var firstTarget = end - firstLength;
   _mergeSort(elements, keyOf, compare, start, middle, elements, firstTarget);

test/algorithms_test.dart

@@ -368,6 +368,19 @@ void main() {
     reverse(l, 0, 6);
     expect(l, equals([2, 1, 4, 3, 6, 5]));
   });
+
+  test('mergeSort works when runtime generic is a subtype of the static type',
+      () {
+    // Regression test for https://github.com/dart-lang/collection/issues/317
+    final length = 1000; // Larger than _mergeSortLimit
+    // Out of order list, with first half guaranteed to empty first during
+    // merge.
+    final list = [
+      for (var i = 0; i < length / 2; i++) -i,
+      for (var i = 0; i < length / 2; i++) i + length,
+    ];
+    expect(() => mergeSort<num>(list), returnsNormally);
+  });
 }
 
 class C {