Extensions: Address follow-ups related to collection expressions #79877

jcouv · 2025-08-12T07:09:24Z

Closes #78960
The first commit addresses minor post-PR-merge follow-ups from last PR: #79702 (comment)

The design needs to be confirmed with LDM, it is outlined here and copied below:

#### Collection expressions

- Extension `Add` works
- Extension `GetEnumerator` works for spread
- Extension `GetEnumerator` does not affect the determination of the element type (must be instance)
- Static `Create` extension methods should not count as a blessed **create** method
- Should extension countable properties affect collection expressions?

#### `params` collections

- Extensions `Add` does not affect what types are allowed with `params`

Note: the last LDM decision regarding pattern-based constructs was to not let extension properties count as "countable properties" in any of the relevant scenarios (list-patterns and implicit indexers) so I kept that behavior here as well.

Relates to test plan #76130

jcouv · 2025-08-12T07:25:04Z

src/Compilers/CSharp/Portable/Binder/Binder_Conversions.cs

+                    {
+                        if (member.TryGetCorrespondingExtensionImplementationMethod() is { } extensionImplementation)
+                        {
+                            member = extensionImplementation;


📝 since we swap the implementation method for the member here and reduce it below, I had to relax the assertion in TypeMap again (see PR)
An alternative would be to implement the logic for this method specially for new extension methods, instead of using the implementation method to leverage the existing handling

AlekseyTs · 2025-08-13T02:25:40Z

src/Compilers/CSharp/Portable/Binder/Binder_Conversions.cs

                {
                    // If the method is generic, skip it if the type arguments cannot be inferred.
                    var member = candidate.Member;
+                    if (member.GetIsNewExtensionMember())


if (member.GetIsNewExtensionMember())

I think this special handling deserves comment. #Closed

AlekseyTs · 2025-08-13T02:29:36Z

src/Compilers/CSharp/Portable/Binder/Binder_Conversions.cs

                builder: out var builder);
            // Collection expression target types require instance method GetEnumerator.
-            if (result && builder.ViaExtensionMethod) // Tracked by https://github.com/dotnet/roslyn/issues/78960: Add test coverage for new extensions
+            if (result && builder.ViaExtensionMethod)


ViaExtensionMethod

Is this flag set when a new extension is used? #Closed

Yes, see getEnumeratorInfo

if (SatisfiesGetEnumeratorPattern(syntax, collectionSyntax, ref builder, collectionExpr, isAsync, viaExtensionMethod: true, diagnostics)) { return createPatternBasedEnumeratorResult(ref builder, collectionExpr, isAsync, viaExtensionMethod: true, diagnostics); }

The relevant test is CollectionExpression_09

AlekseyTs · 2025-08-13T02:35:14Z

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

            {
                return ConsiderEverything;
            }
-            else if (comparison == TypeCompareKind.AllIgnoreOptions)


else if (comparison == TypeCompareKind.AllIgnoreOptions)

I see no harm in keeping this case. Should we switch to a switch, BTW. #Closed

I prefer to remove since not exercised

AlekseyTs · 2025-08-13T02:40:02Z

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

        {
            // mapping contents are read-only hereafter
-            Debug.Assert(allowAlpha || !from.Any(static tp => tp is SubstitutedTypeParameterSymbol));
+            Debug.Assert(allowAlpha || !from.Any(static tp => tp is SubstitutedTypeParameterSymbol && tp.ContainingSymbol is not SourceExtensionImplementationMethodSymbol));


tp.ContainingSymbol is not SourceExtensionImplementationMethodSymbol

I think it would be better to check if the type parameter is original definition. Those should be fine to substitute regardless what the container is and the check would be more robust long term. #Closed

AlekseyTs · 2025-08-13T02:44:30Z

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

            (type, parameter, unused) => type.TypeKind == TypeKind.TypeParameter && (parameter is null || TypeSymbol.Equals(type, parameter, TypeCompareKind.ConsiderEverything2));

-        public static bool ContainsTypeParameter(this TypeSymbol type, MethodSymbol parameterContainer)
+        public static bool ContainsTypeParameter(this TypeSymbol type, Symbol parameterContainer)


parameterContainer

Consider renaming to "typeParameterContainer". Here and in s_isTypeParameterWithSpecificContainerPredicate initialization #Closed

AlekseyTs · 2025-08-13T02:52:56Z

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

-                // (7,26): error CS1929: 'MyCollection<object>' does not contain a definition for 'Add' and the best extension method overload 'E.extension<T>(MyCollection<T>).Add(T)' requires a receiver of type 'MyCollection<T>'
-                // MyCollection<object> c = [oNull, oNotNull];
-                Diagnostic(ErrorCode.ERR_BadInstanceArgType, "[oNull, oNotNull]").WithArguments("MyCollection<object>", "Add", "E.extension<T>(MyCollection<T>).Add(T)", "MyCollection<T>").WithLocation(7, 26));
+        comp.VerifyEmitDiagnostics();


comp.VerifyEmitDiagnostics();

Consider executing and observing the result #Closed

AlekseyTs · 2025-08-13T03:01:05Z

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

+    }
+}
+
+public class D : System.Collections.IEnumerable


D

Is this type used? #Closed

AlekseyTs · 2025-08-13T03:02:14Z

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

+{
+    extension(C c)
+    {
+        public int Length => 2;


2

I assume the purpose of the test is to verify that this property isn't used. Let's throw from it then. #Closed

AlekseyTs · 2025-08-13T03:05:53Z

Done with review pass (commit 2)

jjonescz · 2025-08-13T12:13:53Z

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

+}
+""";
+        var comp = CreateCompilationWithIL(src, ilSrc);
+        comp.VerifyEmitDiagnostics();


Should we verify execution to check that E2 was used, not E? #Resolved

jjonescz · 2025-08-13T12:16:25Z

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

+""";
+        var comp = CreateCompilation(src);
+        comp.VerifyEmitDiagnostics(
+            // (1,7): error CS1921: The best overloaded method match for 'E.extension(C).Add(int)' has wrong signature for the initializer element. The initializable Add must be an accessible instance method.


This error wording seems inaccurate - the Add can be an instance or extension method. But I guess that's a pre-existing issue. #WontFix

Yup. I'll leave as pre-existing issue

AlekseyTs

LGTM (commit 4)

Use CLRSignatureCompareOptions (follow-up from collisions PR)

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jcouv self-assigned this Aug 12, 2025

jcouv added Area-Compilers Feature - Extension Everything The extension everything feature labels Aug 12, 2025

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Extensions: nullability follow-ups #78828

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Extensions: Address follow-ups related to collection expressions

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Test plan for "extensions" feature #76130

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Extensions: Address follow-ups related to collection expressions #79877

Extensions: Address follow-ups related to collection expressions #79877

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