Initial support for union declarations by AlekseyTs · Pull Request #82500 · dotnet/roslyn

AlekseyTs · 2026-02-23T19:05:43Z

Supporting:

union <name> ...

Supporting: - ```union <name> ... ``` - ```record union <name> ... ```

AlekseyTs · 2026-02-23T23:18:06Z

Working group decided to not support record union <name> ... . I will update the PR shortly

AlekseyTs · 2026-02-24T14:17:25Z

@RikkiGibson, @333fred, @dotnet/roslyn-compiler Please review

jcouv · 2026-02-24T19:26:51Z

+                // union S1(int);
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "S1").WithArguments("System.Object").WithLocation(100, 7)
+                );
+        }


Not sure if this already planned for a later PR, consider testing the various modifiers ref union, abstract union, readonly union, unsafe union, ... #Resolved

Also various parameter modifiers for the case types.

This PR is meant to enable the scenario, but doesn't include complete test coverage

333fred · 2026-02-24T20:45:15Z

+            comp.VerifyEmitDiagnostics(
+                // (100,10): error CS1031: Type expected
+                // union S1();
+                Diagnostic(ErrorCode.ERR_TypeExpected, ")").WithLocation(100, 10)


Why is this error reported and not CS9401? #Resolved

Why is this error reported and not CS9401?

Grammar requires at least one type, parser reports accordingly. The CS9401 is for cases when the types list is missing in all partial declarations

It is Ok to omit the list, but if the list is not omitted, there must be at least one type

It feels like we should still be reporting 9401 here. It's odd that the better error text is reported in the scenario where the user typed less.

It feels like we should still be reporting 9401 here.

I am not necessarily sharing your opinion, but I will capture it in a PROTOTYPE comment. I think that the current behavior is sufficiently good for preview

333fred · 2026-02-24T20:56:14Z

+";
+            var comp = CreateCompilation([src, UnionAttributeSource]);
+            comp.VerifyEmitDiagnostics(
+                // (100,15): error CS0111: Type 'S1' already defines a member called 'S1' with the same parameter types


Could we perhaps report a better error here? #Resolved

Could we perhaps report a better error here?

Probably, but it is not a priority at the moment.

333fred · 2026-02-24T20:59:27Z

+                // union S1(int);
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "S1").WithArguments("System.Object").WithLocation(100, 7)
+                );
+        }


Also various parameter modifiers for the case types.

RikkiGibson · 2026-02-25T05:02:12Z

+            Assert.True(members[1].IsImplicitlyDeclared);
+            Assert.False(members[2].IsStatic);
+            Assert.True(members[2].IsImplicitlyDeclared);
+            Assert.False(members[^3].IsStatic);


Is there a particular reason for accessing "from start" sometimes and "from end" other times here? #Resolved

Is there a particular reason for accessing "from start" sometimes and "from end" other times here?

This is in artifact of support for record unions. The test was a theory covering both declaration kinds and for records there were some record specific members in the middle

jcouv · 2026-02-25T17:03:12Z


 struct_declaration
-  : attribute_list* modifier* 'struct' identifier_token type_parameter_list? parameter_list? base_list? type_parameter_constraint_clause* '{'? member_declaration* '}'? ';'?
+  : attribute_list* modifier* ('struct' | 'union') identifier_token type_parameter_list? parameter_list? base_list? type_parameter_constraint_clause* '{'? member_declaration* '}'? ';'?


Passing on feedback from @CyrusNajmabadi in #82526 (comment)

This is very weird that this is part of struct_declaration when it changes that keyword. I would expect a different type-decl here.

FWIW, I had the same reaction. I found it strange to use the parameter list for the union's case list.
#Resolved

The syntax model is a public API and, therefore will go though the standard review process. In this PR, I really don't focus on this aspect. The goal is to enable an ability to use a union declaration in code.

jcouv · 2026-02-25T17:04:12Z

            // For extension declarations, there must be a parameter list
            var paramList = CurrentToken.Kind == SyntaxKind.OpenParenToken || isExtension
-                ? ParseParenthesizedParameterList(forExtension: isExtension) : null;
+                ? ParseParenthesizedParameterList(forExtensionOrUnion: isExtension || isUnion) : null;


Passing on feedback from @CyrusNajmabadi from #82526 (comment)

Imo,this is very difficult to understand precedence here. I would parenthesize for clarity. #Resolved

very difficult to understand precedence here. I would parenthesize for clarity.

The modified condition is already parenthesized:

(forExtensionOrUnion: isExtension || isUnion)

CyrusNajmabadi · 2026-02-26T07:12:43Z

Fwiw, I only noticed this in julien' ide pr, but reusing StructDecl here with a different keyword/kind doesn't fit how I think TypeDecl is intended to be used. Like with RecordDecl, I think this should be it's own TypeDecl subclass. So keep the new UnionDecl syntaxkind, but pair withe a UnionDecl subclass.

If I can be included in the API review, that would be appreciated. Thanks.

@333fred fyi, tnx.

Initial support for union declarations

e6dfacc

Supporting: - ```union <name> ... ``` - ```record union <name> ... ```

AlekseyTs requested review from 333fred and RikkiGibson February 23, 2026 19:05

AlekseyTs requested review from a team as code owners February 23, 2026 19:05

AlekseyTs added Area-Compilers Feature - Unions labels Feb 23, 2026

dotnet-policy-service Bot added the Needs API Review Needs to be reviewed by the API review council label Feb 23, 2026

AlekseyTs added 4 commits February 23, 2026 15:28

Fixup

367b82a

Remove support for record union ...

5d21bda

Fixup

000ed3a

Update generated file

9f1e355

jcouv reviewed Feb 24, 2026

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333fred reviewed Feb 24, 2026

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AlekseyTs requested review from a team and 333fred February 24, 2026 21:38

Fixup

b2ba4bf

333fred approved these changes Feb 25, 2026

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RikkiGibson approved these changes Feb 25, 2026

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jcouv mentioned this pull request Feb 25, 2026

Unions: add basic IDE support #82526

Closed

jcouv reviewed Feb 25, 2026

View reviewed changes

AlekseyTs enabled auto-merge (squash) February 25, 2026 18:44

AlekseyTs merged commit 49a4215 into dotnet:features/Unions Feb 25, 2026
27 of 28 checks passed

This was referenced Mar 7, 2026

[release/11.0.1xx-preview2] Source code updates from dotnet/roslyn dotnet/dotnet#5293

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[release/10.0.3xx] Source code updates from dotnet/roslyn dotnet/dotnet#5292

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[main] Source code updates from dotnet/roslyn dotnet/dotnet#5307

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This was referenced Mar 10, 2026

[Automated] PRs inserted in VS build main-11609.20 #82668

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[Automated] PRs inserted in VS build feature.debugger.wix5-11613.132 #82758

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AlekseyTs commented Feb 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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CyrusNajmabadi commented Feb 26, 2026

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AlekseyTs commented Feb 23, 2026 •

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