Extensions: fix crash with targeted attribute on extension block by jcouv · Pull Request #82463 · dotnet/roslyn

jcouv · 2026-02-19T21:57:53Z

Fixes #82445
Fixes #82444
Relates to feature #76130

Problem: Placing an attribute with a target specifier (such as [return: Obsolete]) on an extension block caused a crash in MatchAttributeTarget. The allowedTargets == None branch assumed only interactive/global symbols (Assembly/Module) could have no allowed targets, and threw on any other DefaultAttributeLocation value.
Extension blocks (which are types with AllowedAttributeLocations == None) hit the throwing branch.

Fix:

Set DefaultAttributeLocation = None for extension types in SourceNamedTypeSymbol
Handled None case in MatchAttributeTarget
Unified the diagnostic reported for attribute on extension block, between the existing check [MyAttribute] extension(int i) and the new check [return: MyAttribute] extension(int i)

We should discuss whether one or both of these scenarios should use a warning instead of an error.

[return: My] // warning CS0657: 'return' is not a valid attribute location for this declaration. Valid attribute locations for this declaration are 'type'. All attributes in this block will be ignored.
[fake: My] // warning CS0658: 'fake' is not a recognized attribute location. Valid attribute locations for this declaration are 'type'. All attributes in this block will be ignored.
class D
{
}

jjonescz · 2026-02-20T09:39:58Z

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs


+                            case AttributeLocation.None:
+                                // No attributes are allowed on this declaration (e.g. extension blocks).
+                                diagnostics.Add(ErrorCode.ERR_AttributeNotAllowedOnExtensionBlock, targetOpt.Identifier.GetLocation());


Looks like the diagnostic is extension-specific. Should we update the comment (e.g. doesn't seem correct) and assert that the symbol is an extension? #Resolved

jjonescz · 2026-02-20T09:45:26Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

        var comp = CreateCompilation(src);
        comp.VerifyEmitDiagnostics(
-            // (3,6): error CS0592: Attribute 'System.Obsolete' is not valid on this declaration type. It is only valid on 'class, struct, enum, constructor, method, property, indexer, field, event, interface, delegate' declarations.
+            // (3,6): error CS9360: Attributes are not allowed on extension blocks.


Is issue #82444 also fixed by this PR? #Resolved

AlekseyTs · 2026-02-20T16:26:47Z

src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol.cs

        AttributeLocation IAttributeTargetSymbol.DefaultAttributeLocation
        {
-            get { return AttributeLocation.Type; }
+            get { return IsExtension ? AttributeLocation.None : AttributeLocation.Type; }


AttributeLocation.None

We probably should add a dedicated location for an extension block #Resolved

AlekseyTs · 2026-02-20T16:29:03Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

+        var model = comp.GetSemanticModel(tree);
+        var type = tree.GetRoot().DescendantNodes().OfType<ExtensionBlockDeclarationSyntax>().Single();
+        var symbol = model.GetDeclaredSymbol(type);
+        AssertEx.SetEqual([], symbol.GetAttributes().Select(a => a.ToString()));


.Select(a => a.ToString())

Is this needed? #Resolved

AlekseyTs · 2026-02-20T16:30:01Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

+        var model = comp.GetSemanticModel(tree);
+        var type = tree.GetRoot().DescendantNodes().OfType<ExtensionBlockDeclarationSyntax>().Single();
+        var symbol = model.GetDeclaredSymbol(type);
+        AssertEx.SetEqual([], symbol.GetAttributes().Select(a => a.ToString()));


.Select(a => a.ToString())

Is this needed? #Resolved

AlekseyTs · 2026-02-20T16:30:46Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

+        var model = comp.GetSemanticModel(tree);
+        var type = tree.GetRoot().DescendantNodes().OfType<ExtensionBlockDeclarationSyntax>().Single();
+        var symbol = model.GetDeclaredSymbol(type);
+        AssertEx.SetEqual([], symbol.TypeParameters[0].GetAttributes().Select(a => a.ToString()));


.Select(a => a.ToString())

Is this needed? #Resolved

AlekseyTs · 2026-02-20T16:31:33Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

+        var model = comp.GetSemanticModel(tree);
+        var type = tree.GetRoot().DescendantNodes().OfType<ExtensionBlockDeclarationSyntax>().Single();
+        var symbol = model.GetDeclaredSymbol(type);
+        AssertEx.SetEqual([], symbol.GetAttributes().Select(a => a.ToString()));


.Select(a => a.ToString())

Is this needed? #Resolved

AlekseyTs · 2026-02-20T16:32:36Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

+        comp.VerifyEmitDiagnostics(
+            // (3,6): warning CS0658: 'fake' is not a recognized attribute location. Valid attribute locations for this declaration are ''. All attributes in this block will be ignored.
+            //     [fake: My]
+            Diagnostic(ErrorCode.WRN_InvalidAttributeLocation, "fake").WithArguments("fake", "").WithLocation(3, 6));


WRN_InvalidAttributeLocation

I think it would be better to report ERR_AttributeNotAllowedOnExtensionBlock here as well #Resolved

AlekseyTs · 2026-02-20T16:33:12Z

Done with review pass (commit 1)

Extensions: fix crash with targeted attribute on extension block

e277bc8

jcouv self-assigned this Feb 19, 2026

jcouv added Area-Compilers Feature - Extension Everything The extension everything feature labels Feb 19, 2026

jcouv marked this pull request as ready for review February 20, 2026 01:16

jcouv requested a review from a team as a code owner February 20, 2026 01:16

jjonescz reviewed Feb 20, 2026

View reviewed changes

AlekseyTs reviewed Feb 20, 2026

View reviewed changes

Julien Couvreur added 2 commits February 20, 2026 22:37

Address feedback and simplify change

e963038

Tweaks

a4a7d89

jcouv requested review from AlekseyTs and jjonescz February 21, 2026 06:41

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Extensions: fix crash with targeted attribute on extension block#82463

Extensions: fix crash with targeted attribute on extension block#82463
jcouv wants to merge 3 commits intodotnet:mainfrom
jcouv:extension-attribute

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