Remove Try-Both matching mode for unions. by AlekseyTs · Pull Request #84897 · dotnet/roslyn

AlekseyTs · 2026-08-15T03:49:19Z

See dotnet/csharplang#10302.
Closes #84883.

Microsoft Reviewers: Open in CodeFlow

…r in commit 8c386fe

…enarios" This reverts only compiler changes from commit 1979ab1.

See dotnet/csharplang#10302. Closes dotnet#84883.

azure-pipelines · 2026-08-15T03:49:36Z

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Copilot

Pull request overview

This PR updates the C# compiler’s union pattern-matching implementation to remove the “try-both” (instance-or-value) matching mode, aligning diagnostics and bound-tree representation with the new behavior and adjusting tests/resources accordingly (see issue #84883 / csharplang #10302).

Changes:

Simplifies union-matching lowering by removing the “instance arm” and representing union matching as a single value-pattern rewrite.
Refactors bound nodes and binder plumbing from UnionMatchingMode (for most patterns) to a boolean IsUnionMatching.
Updates CS8780 wording and expected diagnostics across pattern-matching and flow tests, plus localized resource strings.

Reviewed changes

Copilot reviewed 40 out of 42 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/Emit3/Semantics/PatternMatchingTests3.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/Emit3/Semantics/PatternMatchingTests_NullableTypes.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/Emit3/Semantics/PatternMatchingTests_ListPatterns.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/Emit3/FlowAnalysis/PatternsVsRegions.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/Emit3/FlowAnalysis/FlowTests.cs	Updates CS8780 message expectations
src/Compilers/CSharp/Test/CSharp15/ClosedClassesTests.cs	Updates diagnostics for union/closed-class scenarios
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf	Updates localized CS8780 message
src/Compilers/CSharp/Portable/CSharpResources.resx	Updates CS8780 message text
src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_IsOperator.cs	Adjusts constant folding call shape
src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_AsOperator.cs	Adjusts constant folding call shape
src/Compilers/CSharp/Portable/Generated/BoundNodes.xml.Generated.cs	Regenerates bound-node APIs for new fields
src/Compilers/CSharp/Portable/BoundTree/BoundRelationalPattern.cs	Updates validation to `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundRecursivePattern.cs	Updates validation to `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundPatternWithUnionMatching.cs	Simplifies union-matching pattern shape
src/Compilers/CSharp/Portable/BoundTree/BoundPattern.cs	Replaces `UnionMatchingMode` with `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml	Updates bound-node schema for union matching
src/Compilers/CSharp/Portable/BoundTree/BoundNegatedPattern.cs	Updates validation to `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundListPattern.cs	Updates validation to `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundITuplePattern.cs	Updates validation to `IsUnionMatching`
src/Compilers/CSharp/Portable/BoundTree/BoundConstantPattern.cs	Defines `IsUnionMatching` based on mode
src/Compilers/CSharp/Portable/Binder/UnionMatchingRewriter.cs	Rewrites union patterns as value-property patterns
src/Compilers/CSharp/Portable/Binder/SwitchExpressionArmBinder.cs	Updates binder call to new signature
src/Compilers/CSharp/Portable/Binder/SwitchBinder.cs	Updates binder call to new signature
src/Compilers/CSharp/Portable/Binder/SwitchBinder_Patterns.cs	Updates binder call to new signature
src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder.cs	Updates asserts to `IsUnionMatching`
src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder_CheckOrReachability.cs	Updates pattern rewrites to new fields
src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs	Refactors union matching type-compat checks
src/Compilers/CSharp/Portable/Binder/Binder_Operators.cs	Refactors is/as constant analysis helpers

Copilot

Pull request overview

Copilot reviewed 40 out of 42 changed files in this pull request and generated no new comments.

Suppressed comments (3)

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml:2475

The BoundNodes.xml doc comment has a grammatical error: it refers to two fields (ValueProperty and ValuePattern) but uses the singular verb "represents".

      The ValueProperty and ValuePattern represents a Union matching operation
      pending a rewrite to a BoundRecursivePattern.

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml:2491

The BoundNodes.xml doc comment refers to "Pattern.NarrowedType", but the field is named ValuePattern; this makes the comment ambiguous/inaccurate.

      NarrowedType is Pattern.NarrowedType

src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs:1928

This PR changes union pattern binding semantics (removing Try-Both matching) to address #84883, but there doesn't appear to be a targeted regression test covering the reported scenario (open generic union + struct case + variable designation in the case pattern). Without a dedicated test, it's easy for future refactors to reintroduce CS8780 in that corner case.

Consider adding a compiler test based on the issue repro (e.g., in C#15 union pattern tests) asserting that ValUnion<T> u => u switch { ValCase(var v) => v } compiles without CS8780.

            NamedTypeSymbol? unionValueMatchingInputType = PrepareForUnionMatchingIfAppropriateAndReturnUnionMatchingInputType(node, ref inputType, ref unionType, diagnostics);
            CompoundUseSiteInfo<AssemblySymbol> useSiteInfo = GetNewCompoundUseSiteInfo(diagnostics);
            hasErrors |= CheckValidPatternType(typeSyntax, inputType, patternType.Type, diagnostics: diagnostics, ref useSiteInfo, out _) ||
                         (unionType is not null && CheckValidPatternTypeForUnionCases(typeSyntax, unionType, patternType.Type, diagnostics: diagnostics, ref useSiteInfo));
            diagnostics.Add(typeSyntax, useSiteInfo);

            isUnionMatching = unionValueMatchingInputType is not null;

AlekseyTs · 2026-08-15T04:17:10Z

@RikkiGibson, @jjonescz, @333fred Please review

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Pull request overview

Copilot reviewed 40 out of 42 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs:1918

This PR is marked as closing #84883, but I couldn’t find a compiler regression test that exercises the reported scenario (open generic union + struct case + pattern variable designation should not produce CS8780). Please add a targeted test (likely under src/Compilers/CSharp/Test/CSharp15/UnionsTests.cs) that uses the repro from the issue and asserts the code compiles with no diagnostics.

        private void CheckCompatibilityOfInputWithPatternType(
            SyntaxNode node,
            ExpressionSyntax typeSyntax,
            NamedTypeSymbol? unionType,
            TypeSymbol inputType,
            BoundTypeExpression patternType,
            ref bool hasErrors,
            BindingDiagnosticBag diagnostics,
            out bool isUnionMatching)

jjonescz · 2026-08-17T09:28:02Z

Do we have a test for #84883, i.e., a recursive pattern like the following?

record struct ValCase(int Value); union ValUnion<T>(ValCase); static int FromVal<T>(ValUnion<T> u) => u switch { ValCase(var v) => v };

No, we don't. There are existing tests that observe the fact that the union instance isn't matched

AlekseyTs · 2026-08-17T17:55:17Z

@RikkiGibson, @333fred For a second review

* upstream/main: (730 commits) Improve recovery for repeated partial type modifiers (#84934) Add standalone C# LSP telemetry (#84874) Suppress AI artifact audit failure issues (#84925) Remove empty ExternalAccessAspNetCoreResources.resx (#84939) Update helix job monitor version (#84940) Add HasPendingUpdates to HotReloadService.Updates to be used in dotnet-watch (#84891) Add LocalizableBranches parameter for OneLocBuild (#84933) Pin version of Microsoft.CodeAnalysis.Analyzers (#84924) Centralize record and union keyword checks (#84928) Caching compiler: support binary additional texts (#84916) Remove unused AdditionalTextComparer (#84917) Remove Try-Both matching mode for unions. (#84897) Update CodeStyleAnalyzerVersion to 5.9.0 (#84919) Fix/84847 source generated rename conflict (#84849) Return MethodNotFound for unsupported LSP method dispatch (#84892) [main] Update dependencies from dotnet/arcade (#84896) [main] Update dependencies from dotnet/arcade (#84882) Track status for feature "Type Parameter Inference from Constraints" (#84869) [main] Source code updates from dotnet/dotnet (#84879) Stop labeling Loc PRs as community (#84878) ...

AlekseyTs added 3 commits August 14, 2026 05:51

Revert wording changes made for ERR_DesignatorBeneathPatternCombinato…

96acc6a

…r in commit 8c386fe

Revert "Make GetIsOperatorConstantResult more accurate for generic sc…

1578ca8

…enarios" This reverts only compiler changes from commit 1979ab1.

Remove Try-Both matching mode for unions.

3e95083

See dotnet/csharplang#10302. Closes dotnet#84883.

AlekseyTs added the Area-Compilers label Aug 15, 2026

AlekseyTs requested a review from a team as a code owner August 15, 2026 03:49

AlekseyTs added the Feature - Unions label Aug 15, 2026

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Comment thread src/Compilers/CSharp/Portable/Binder/UnionMatchingRewriter.cs

Fixup

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jjonescz approved these changes Aug 17, 2026

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AlekseyTs merged commit 164ff4b into dotnet:main Aug 18, 2026
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This was referenced Aug 18, 2026

[release/11.0.1xx] Source code updates from dotnet/roslyn dotnet/dotnet#8411

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[main] Source code updates from dotnet/roslyn dotnet/dotnet#8427

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davidwengier mentioned this pull request Aug 24, 2026

Update roslyn to 5.11.0-1.26424.9 dotnet/vscode-csharp#9701

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jjonescz added this to the 18.11 milestone Aug 25, 2026

AlekseyTs mentioned this pull request Sep 8, 2026

Always match union payloads through the Value property pattern dotnet/runtime#131879

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robertodalmonte mentioned this pull request Sep 11, 2026

Union types reference: pet is Pet note is wrong, and the null-handling sample implies a warning that never appears dotnet/docs#55951

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RikkiGibson mentioned this pull request Sep 11, 2026

C# compiler should not report CS8780 on receiving variable in switch on union w/ generic type #85270

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externl mentioned this pull request Sep 30, 2026

Map Slice variant enums to C# unions icerpc/icerpc-csharp#5004

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Remove Try-Both matching mode for unions. - #84897

Remove Try-Both matching mode for unions.#84897
AlekseyTs merged 5 commits into
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