Extensions: Add ReduceExtensionMember API by jcouv · Pull Request #80659 · dotnet/roslyn

jcouv · 2025-10-11T20:43:29Z

Closes #80273 (pending API review to decide whether to have an API on ISymbol or split as implemented here)

There is a small change in behavior when dealing with extensions that aren't fully inferred. Previously, GetCompatibleSubstitutedMember dropped them. Now, it's up to the caller to decide.
The LookupSymbols_WasNotFullyInferred test illustrates such a change. For lookup, we do want to return members that can't be fully inferred.

Relates to test plan #76130

AlekseyTs · 2025-10-16T03:37:58Z

src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs

        IMethodSymbol? ReduceExtensionMethod(ITypeSymbol receiverType);

+        /// <summary>
+        /// If this is a (new) extension method that can be applied to a receiver of the given type,


(new) extension method

"method of an extension block"? #Closed

AlekseyTs · 2025-10-16T03:46:49Z

src/Compilers/CSharp/Portable/Symbols/PublicModel/MethodSymbol.cs

+            if (_underlying.GetIsNewExtensionMember() && SourceMemberContainerTypeSymbol.IsAllowedExtensionMember(_underlying))
+            {
+                var csharpReceiver = receiverType.EnsureCSharpSymbolOrNull(nameof(receiverType));
+                return (IMethodSymbol?)SourceNamedTypeSymbol.GetCompatibleSubstitutedMember(compilation: null, _underlying, csharpReceiver, wasExtensionFullyInferred: out _).GetPublicSymbol();


wasExtensionFullyInferred

Should we be checking this value? #Closed

I don't think so. See description in OP: "For lookup, we do want to return members that can't be fully inferred."

AlekseyTs · 2025-10-16T03:47:40Z

src/Compilers/CSharp/Portable/Symbols/PublicModel/PropertySymbol.cs

+            if (_underlying.GetIsNewExtensionMember() && SourceMemberContainerTypeSymbol.IsAllowedExtensionMember(_underlying))
+            {
+                var csharpReceiver = receiverType.EnsureCSharpSymbolOrNull(nameof(receiverType));
+                return (IPropertySymbol?)SourceNamedTypeSymbol.GetCompatibleSubstitutedMember(compilation: null, _underlying, csharpReceiver, wasExtensionFullyInferred: out _).GetPublicSymbol();


wasExtensionFullyInferred

Should we be checking this value? #Closed

Answered elsewhere

AlekseyTs · 2025-10-16T16:47:12Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+
+        INamedTypeSymbol systemObject = comp.GetSpecialType(SpecialType.System_Object).GetPublicSymbol();
+
+        AssertEx.Equal("void E.<G>$66F77D1E46F965A5B22D4932892FA78B<T>.M()", method.ToTestDisplayString());


method.ToTestDisplayString()

I think for these tests it would be better to use default display style with "extension" instead of the grouping name. #Closed

AlekseyTs · 2025-10-16T16:50:45Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+    }
+
+    [Fact]
+    public void ReduceExtensionMember_12()


ReduceExtensionMember_12

It doesn't look like this test is trying to use ReduceExtensionMember APIs. #Closed

That's correct, this is testing the internal API which has a different name, but is morally equivalent. I don't think this warrants a different test name. An alternative would be to rename the internal API to use the same name as the public one.

AlekseyTs · 2025-10-16T16:50:53Z

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

+    }
+
+    [Fact]
+    public void ReduceExtensionMember_13()


ReduceExtensionMember_13

It doesn't look like this test is trying to use ReduceExtensionMember APIs. #Closed

Asnwered above

AlekseyTs · 2025-10-16T16:52:45Z

Done with review pass (Commit 1) #Closed

AlekseyTs

LGTM (commit 4)

jcouv · 2025-10-20T17:24:14Z

@RikkiGibson @CyrusNajmabadi for second review. Thanks

jcouv · 2025-10-21T17:12:44Z

@RikkiGibson @CyrusNajmabadi for second review. Thanks

CyrusNajmabadi · 2025-10-21T17:44:36Z

src/Compilers/Core/Portable/Symbols/IMethodSymbol.cs

+        /// <summary>
+        /// If this is a method of an extension block that can be applied to a receiver of the given type,
+        /// returns the method symbol in the substituted extension for that receiver type. Otherwise, returns null.
+        /// </summary>


i think it would be really good to give an explicit code example of what this is versus ReduceExtensionMethod

I'll do a follow-up PR for this

jcouv self-assigned this Oct 11, 2025

jcouv added Area-Compilers Feature - Extension Everything The extension everything feature labels Oct 11, 2025

dotnet-policy-service bot added VSCode Needs API Review Needs to be reviewed by the API review council labels Oct 11, 2025

jcouv mentioned this pull request Oct 11, 2025

Extension members not suggested by IntelliSense when "Show items from unimported namespaces" is enabled #80561

Closed

jcouv force-pushed the extensions-reduce branch 4 times, most recently from bd1e5af to cadd91d Compare October 13, 2025 14:53

jcouv mentioned this pull request Oct 14, 2025

MethodTypeInferrer.Fix has potential exposure to null compilation #80658

Open

Extensions: Add ReduceExtensionMember API

a151018

jcouv force-pushed the extensions-reduce branch from cadd91d to a151018 Compare October 14, 2025 18:21

jcouv marked this pull request as ready for review October 14, 2025 20:11

jcouv requested review from a team as code owners October 14, 2025 20:11

jcouv requested review from AlekseyTs and jjonescz October 15, 2025 17:42

AlekseyTs reviewed Oct 16, 2025

View reviewed changes

Julien Couvreur added 3 commits October 16, 2025 14:09

Address feedback

8f90cc4

Rename internal API

609150c

Use "extension" symbol display

db8cc41

AlekseyTs approved these changes Oct 16, 2025

View reviewed changes

Merge remote-tracking branch 'dotnet/main' into extensions-reduce

6f5425f

jcouv requested review from CyrusNajmabadi and RikkiGibson October 16, 2025 22:54

CyrusNajmabadi reviewed Oct 21, 2025

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CyrusNajmabadi approved these changes Oct 21, 2025

View reviewed changes

jcouv merged commit 91e1fa9 into dotnet:main Oct 22, 2025
28 checks passed

jcouv deleted the extensions-reduce branch October 22, 2025 19:56

dotnet-policy-service bot added this to the Next milestone Oct 22, 2025

dotnet-maestro bot mentioned this pull request Oct 23, 2025

[main] Source code updates from dotnet/roslyn dotnet/dotnet#3061

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This was referenced Oct 25, 2025

[Automated] PRs inserted in VS build feature.polytype-11124.148 #80896

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[Automated] PRs inserted in VS build main-11124.32 #80897

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[Automated] PRs inserted in VS build feature.debugger.main-11127.165 #80926

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dotnet-maestro bot mentioned this pull request Oct 29, 2025

[main] Source code updates from dotnet/roslyn dotnet/dotnet#3147

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dotnet-maestro bot mentioned this pull request Dec 18, 2025

[main] Source code updates from dotnet/roslyn dotnet/dotnet#3957

Merged

davidwengier modified the milestones: Next, 18.3 Jan 6, 2026


		INamedTypeSymbol systemObject = comp.GetSpecialType(SpecialType.System_Object).GetPublicSymbol();

		AssertEx.Equal("void E.<G>$66F77D1E46F965A5B22D4932892FA78B<T>.M()", method.ToTestDisplayString());

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