Avoid setRange with potentially incompatible types

Fixes #317

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.
 
 ## 1.18.0
 

lib/src/algorithms.dart

@@ -394,8 +394,9 @@ void _merge<E, K>(
   }
   // First list empties first. Reached by break above.
   target[targetOffset++] = secondElement;
-  target.setRange(
-      targetOffset, targetOffset + (secondEnd - cursor2), secondList, cursor2);
+  for (var i = targetOffset; i < targetOffset + (secondEnd - cursor2); i++) {
+    target[i] = secondList[(i - targetOffset) + cursor2];
+  }
 }
 
 /// Sort [elements] using a quick-sort algorithm.

test/algorithms_test.dart

@@ -366,6 +366,15 @@ 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 = 32; // _mergeSortLimit
+    // In order list, first half empties first during merge.
+    final list = List<int>.generate(length, (i) => i);
+    expect(() => mergeSort<num>(list), returnsNormally);
+  });
 }
 
 class C {