Validate presence of nesting selector.

src/Avalonia.Base/Styling/ChildSelector.cs

@@ -65,5 +65,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/DescendentSelector.cs

@@ -70,5 +70,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -25,5 +25,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => true;
     }
 }

src/Avalonia.Base/Styling/NotSelector.cs

@@ -67,5 +67,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _argument.HasValidNestingSelector();
     }
 }

src/Avalonia.Base/Styling/NthChildSelector.cs

@@ -105,6 +105,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         public override string ToString()
         {

src/Avalonia.Base/Styling/OrSelector.cs

@@ -104,6 +104,19 @@ namespace Avalonia.Styling
 
         protected override Selector? MovePrevious() => null;
 
+        internal override bool HasValidNestingSelector()
+        {
+            foreach (var selector in _selectors)
+            {
+                if (!selector.HasValidNestingSelector())
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
         private Type? EvaluateTargetType()
         {
             Type? result = null;

src/Avalonia.Base/Styling/PropertyEqualsSelector.cs

@@ -90,6 +90,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         internal static bool Compare(Type propertyType, object? propertyValue, object? value)
         {

src/Avalonia.Base/Styling/Selector.cs

@@ -86,6 +86,8 @@ namespace Avalonia.Styling
         /// </summary>
         protected abstract Selector? MovePrevious();
 
+        internal abstract bool HasValidNestingSelector();
+
         private static SelectorMatch MatchUntilCombinator(
             IStyleable control,
             Selector start,

src/Avalonia.Base/Styling/Style.cs

@@ -186,8 +186,9 @@ namespace Avalonia.Styling
             if (parent?.Selector is not null)
             {
                 if (Selector is null)
-                    throw new InvalidOperationException("Nested styles must have a selector.");
-                // TODO: Validate that selector contains & in the right place.
+                    throw new InvalidOperationException("Child styles must have a selector.");
+                if (!Selector.HasValidNestingSelector())
+                    throw new InvalidOperationException("Child styles must have a nesting selector.");
             }
 
             Parent = parent;

src/Avalonia.Base/Styling/StyleChildren.cs

@@ -11,23 +11,23 @@ namespace Avalonia.Styling
 
         protected override void InsertItem(int index, IStyle item)
         {
-            base.InsertItem(index, item);
             (item as Style)?.SetParent(_owner);
+            base.InsertItem(index, item);
         }
 
         protected override void RemoveItem(int index)
         {
             var item = Items[index];
+            (item as Style)?.SetParent(null);
             if (_owner.Owner is IResourceHost host)
                 (item as IResourceProvider)?.RemoveOwner(host);
-            (item as Style)?.SetParent(null);
             base.RemoveItem(index);
         }
 
         protected override void SetItem(int index, IStyle item)
         {
-            base.SetItem(index, item);
             (item as Style)?.SetParent(_owner);
+            base.SetItem(index, item);
             if (_owner.Owner is IResourceHost host)
                 (item as IResourceProvider)?.AddOwner(host);
         }

src/Avalonia.Base/Styling/TemplateSelector.cs

@@ -49,5 +49,6 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => null;
+        internal override bool HasValidNestingSelector() => _parent?.HasValidNestingSelector() ?? false;
     }
 }

src/Avalonia.Base/Styling/TypeNameAndClassSelector.cs

@@ -140,6 +140,7 @@ namespace Avalonia.Styling
         }
 
         protected override Selector? MovePrevious() => _previous;
+        internal override bool HasValidNestingSelector() => _previous?.HasValidNestingSelector() ?? false;
 
         private string BuildSelectorString()
         {

tests/Avalonia.Base.UnitTests/Styling/SelectorTests_Nesting.cs

@@ -219,6 +219,48 @@ namespace Avalonia.Base.UnitTests.Styling
             Assert.Throws<InvalidOperationException>(() => nested.Selector.Match(control, parent));
         }
 
+        [Fact]
+        public void Adding_Child_With_No_Nesting_Selector_Fails()
+        {
+            var control = new Control1();
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => x.Class("foo"));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Adding_Combinator_Selector_Child_With_No_Nesting_Selector_Fails()
+        {
+            var control = new Control1();
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => x.Class("foo").Descendant().Class("bar"));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Adding_Or_Selector_Child_With_No_Nesting_Selector_Fails()
+        {
+            var control = new Control1();
+            var parent = new Style(x => x.OfType<Control1>());
+            var child = new Style(x => Selectors.Or(
+                x.Nesting().Class("foo"),
+                x.Class("bar")));
+
+            Assert.Throws<InvalidOperationException>(() => parent.Children.Add(child));
+        }
+
+        [Fact]
+        public void Can_Add_Child_Without_Nesting_Selector_To_Style_Without_Selector()
+        {
+            var control = new Control1();
+            var parent = new Style();
+            var child = new Style(x => x.Class("foo"));
+
+            parent.Children.Add(child);
+        }
+
         public class Control1 : Control
         {
         }