Stricter key clash detection. Fixes #12691 (#12837)

Author: SteveSandersonMS

src/Components/Components/src/RenderTree/RenderTreeDiffBuilder.cs

@@ -99,6 +99,10 @@ private static void AppendDiffEntriesForRange(
                     if (oldKey != null || newKey != null)
                     {
                         #region "Get diff action by matching on key"
+                        // Regardless of whether these two keys match, since you are using keys, we want to validate at this point that there are no clashes
+                        // so ensure we've built the dictionary that will be used for lookups if any don't match
+                        keyedItemInfos ??= BuildKeyToInfoLookup(diffContext, origOldStartIndex, oldEndIndexExcl, origNewStartIndex, newEndIndexExcl);
+
                         if (Equals(oldKey, newKey))
                         {
                             // Keys match
@@ -108,11 +112,6 @@ private static void AppendDiffEntriesForRange(
                         else
                         {
                             // Keys don't match
-                            if (keyedItemInfos == null)
-                            {
-                                keyedItemInfos = BuildKeyToInfoLookup(diffContext, origOldStartIndex, oldEndIndexExcl, origNewStartIndex, newEndIndexExcl);
-                            }
-
                             var oldKeyItemInfo = oldKey != null ? keyedItemInfos[oldKey] : new KeyedItemInfo(-1, -1);
                             var newKeyItemInfo = newKey != null ? keyedItemInfos[newKey] : new KeyedItemInfo(-1, -1);
                             var oldKeyIsInNewTree = oldKeyItemInfo.NewIndex >= 0;

src/Components/Components/test/RenderTreeDiffBuilderTest.cs

@@ -360,6 +360,23 @@ public void RejectsClashingKeysInNewTree()
             Assert.Equal("More than one sibling has the same key value, 'key1'. Key values must be unique.", ex.Message);
         }
 
+        [Fact]
+        public void RejectsClashingKeysEvenIfAllPairsMatch()
+        {
+            // This sort of scenario would happen if you accidentally used a constant value for @key
+
+            // Arrange
+            AddWithKey(oldTree, "key1", "attrib1a");
+            AddWithKey(oldTree, "key1", "attrib1b");
+
+            AddWithKey(newTree, "key1", "attrib1a");
+            AddWithKey(newTree, "key1", "attrib1b");
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() => GetSingleUpdatedComponent());
+            Assert.Equal("More than one sibling has the same key value, 'key1'. Key values must be unique.", ex.Message);
+        }
+
         [Fact]
         public void HandlesInsertionOfUnkeyedItemsAroundKey()
         {