Keep a fallback component's derived tag helpers during discovery

src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.Helpers.cs

@@ -37,6 +37,28 @@ internal static string StripGenericArity(string typeName)
             return baseName.Length == typeName.Length ? typeName : baseName.ToString();
         }
 
+        /// <summary>
+        /// Returns the name of the component type a descriptor belongs to. A component descriptor's owning
+        /// type is itself, but descriptors derived from a component -- child content and bind, for example --
+        /// carry the owning component's namespace and identifier while their own <see cref="TagHelperDescriptor.TypeName"/>
+        /// is suffixed (e.g. <c>Ns.Card.Header</c> for the <c>Header</c> child content of <c>Ns.Card</c>).
+        /// Reconstructing the owning name from the namespace and identifier lets ownership filtering keep or
+        /// exclude a fallback component and all its derived descriptors together, rather than only the
+        /// component descriptor whose <see cref="TagHelperDescriptor.TypeName"/> matches exactly.
+        /// </summary>
+        internal static string GetOwningTypeName(TagHelperDescriptor descriptor)
+        {
+            var identifier = descriptor.TypeNameIdentifier;
+            if (identifier is null)
+            {
+                return descriptor.TypeName;
+            }
+
+            return descriptor.TypeNamespace is { Length: > 0 } typeNamespace
+                ? typeNamespace + "." + identifier
+                : identifier;
+        }
+
         /// <summary>
         /// Returns the hint name for the decl half of a Razor component generated source given
         /// the impl half's hint name. The decl file substitutes <c>.decl.g.cs</c> for the

src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/SourceGenerators/RazorSourceGenerator.cs

@@ -222,7 +222,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                         return TagHelperCollection.Empty;
                     }
 
-                    return all.Where(fallbackTypeNames, static (descriptor, names) => names.Contains(StripGenericArity(descriptor.TypeName)));
+                    return all.Where(fallbackTypeNames, static (descriptor, names) => names.Contains(StripGenericArity(GetOwningTypeName(descriptor))));
                 })
                 .WithLambdaComparer(static (a, b) => a!.SequenceEqual(b!))
                 .WithTrackingName("SlowTagHelpers");
@@ -239,7 +239,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                     var ((fast, slow), fallbackTypeNames) = pair;
                     var fastOwned = fallbackTypeNames.IsEmpty
                         ? fast
-                        : fast.Where(fallbackTypeNames, static (descriptor, names) => !names.Contains(StripGenericArity(descriptor.TypeName)));
+                        : fast.Where(fallbackTypeNames, static (descriptor, names) => !names.Contains(StripGenericArity(GetOwningTypeName(descriptor))));
                     return TagHelperCollection.Merge(fastOwned, slow);
                 })
                 .WithTrackingName("TagHelpersFromCompilation");

src/Razor/src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.UnitTests/RazorSourceGeneratorComponentTests.cs

@@ -1329,6 +1329,52 @@ public class MyGrid<T> : ComponentBase
         Assert.Equal(2, result.GeneratedSources.Length);
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/84817")]
+    public async Task Component_FallbackComponent_ChildContentParameters()
+    {
+        // A fallback component (unsplittable via @inherits) exposes RenderFragment child-content
+        // parameters. A consumer referencing them as child elements must resolve them, i.e. no
+        // RZ10012 "unexpected name" for Header/ChildContent.
+        var project = CreateTestProject(new()
+        {
+            ["Shared/Card.razor"] = """
+                @inherits EmptyBase
+
+                <header>@Header</header>
+                <main>@ChildContent</main>
+
+                @code {
+                    [Parameter]
+                    public RenderFragment? Header { get; set; }
+
+                    [Parameter]
+                    public RenderFragment? ChildContent { get; set; }
+                }
+                """,
+            ["Shared/Consumer.razor"] = """
+                <Card>
+                    <Header>Expected header</Header>
+                    <ChildContent>Expected content</ChildContent>
+                </Card>
+                """,
+        }, new()
+        {
+            ["EmptyBase.cs"] = """
+                using Microsoft.AspNetCore.Components;
+
+                public abstract class EmptyBase : ComponentBase;
+                """,
+        });
+        var compilation = await project.GetCompilationAsync();
+        var driver = await GetDriverAsync(project);
+
+        // Act
+        var result = RunGenerator(compilation!, ref driver);
+
+        // Assert -- no RZ10012 for the fallback component's child-content parameters
+        result.Diagnostics.Verify();
+    }
+
     [Fact]
     public async Task Component_WithImplicitContext_NestedInWrapper()
     {