Extensions: relax inferrability rule #78758

jcouv · 2025-05-29T20:42:12Z

From discussion with WG, relaxing this restriction allows for greater portability of classic extension methods. But we're keeping the check for members that can't be called with explicit type arguments.
~~To be confirmed with LDM next week.~~

New rule: https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-04.md#extension-declaration-validation

Closes #78472

Relates to test plan #76130

jcouv · 2025-05-30T22:20:42Z

src/Compilers/CSharp/Portable/Symbols/Extensions/SynthesizedExtensionMarker.cs

                return parameter;
            }
-
-            static void checkUnderspecifiedGenericExtension(ParameterSymbol parameter, ImmutableArray<TypeParameterSymbol> typeParameters, BindingDiagnosticBag diagnostics)


📝 ~~moved this logic to SourceNamedTypeSymbol.AfterMembersCompletedChecks as the added GetMembers call was causing a circularity issue~~ (out-of-date after new LDM decision)

AlekseyTs · 2025-06-02T14:42:19Z

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

+
+            void checkUnderspecifiedGenericExtension(ParameterSymbol parameter, ImmutableArray<TypeParameterSymbol> typeParameters, BindingDiagnosticBag diagnostics)
+            {
+                if (GetMembers().All((m, a) => m is MethodSymbol { MethodKind: MethodKind.Ordinary }, arg: (object?)null))


All

Is there an overload that doesn't need unused additional argument? Perhaps we have Any instead? #Closed

AlekseyTs · 2025-06-02T14:46:33Z

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

+
+                if (TryGetOrCreateExtensionMarker() is { Parameters: [var extensionParameter] })
+                {
+                    checkUnderspecifiedGenericExtension(extensionParameter, TypeParameters, diagnostics);


checkUnderspecifiedGenericExtension

I think that long term it would be better to perform this check per member and report an error per member. For example, once we enable indexers, I can imagine that some indexer declarations (referencing remaining type arguments in their parameters) will be allowed, and others won't be allowed. #Closed

AlekseyTs · 2025-06-02T14:56:06Z

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

+
+            void checkUnderspecifiedGenericExtension(ParameterSymbol parameter, ImmutableArray<TypeParameterSymbol> typeParameters, BindingDiagnosticBag diagnostics)
+            {
+                if (GetMembers().All((m, a) => m is MethodSymbol { MethodKind: MethodKind.Ordinary }, arg: (object?)null))


MethodKind.Ordinary

I think operators should be allowed too. At least, consider leaving a comment to follow up, the one that starts with a link to the test plan. Operators are being worked on as we speak. BTW, similar to indexers, I think that some operator declarations would be allowed and other operator declarations wouldn't be allowed. This is even stronger reason to moving to per member check/reporting. #Closed

The criteria we last discussed in WG is whether it's possible to use the member with explicit type arguments.
So I don't think we'll allow either indexers or operators in non-inferrable extension blocks.

FWIW, here's the update I captured in the spec (to be reviewed in LDM this week):

__Inferrability:__ All the type parameters of an extension block must be used in the receiver type when the extension block contains a non-method member. This makes it always possible to infer the type arguments when applied to a receiver of the given receiver type and the member doesn't allow explicit type arguments.

I do see that in the summary, but, to be honest, I do not remember us having a serious discussion about the criteria to the point that I must have forgotten us deciding on having the restriction. At the same time, from our "Tensors and extension operators" meeting I vaguely remember us expressing a desire to support the following scenario from #78472 (comment):

extension<TTensor, TScalar>(TTensor) where TTensor : IReadOnlyTensor<TTensor, TScalar> where TScalar : IAdditionOperators<TScalar, TScalar, TScalar> { public static TTensor operator +(TTensor left, TScalar right); }

Here is a link to the specific summary message from Tanner specifically mentioning the following shape as planned for .NET 10:

extension<TTensor, TScalar>(TTensor) where TTensor : IReadOnlyTensor<TTensor, TScalar> where TScalar : IAdditionOperators<TScalar, TScalar, TScalar> { public static TTensor operator +(TTensor left, TScalar right); }

AlekseyTs · 2025-06-02T15:02:05Z

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

+
+        var comp = CreateCompilation(src);
+        comp.VerifyEmitDiagnostics(
+            // (1,9): error CS9286: 'object' does not contain a definition for 'M' and no accessible extension member 'M' for receiver of type 'object' could be found (are you missing a using directive or an assembly reference?)


// (1,9): error CS9286: 'object' does not contain a definition for 'M' and no accessible extension member 'M' for receiver of type 'object' could be found (are you missing a using directive or an assembly reference?)

Are we tracking an improvement for diagnostics in this scenarios? #Closed

Yes, we have a general tracking comment in ResolveExtensions: "// Tracked by #76130 : we'll want to describe what went wrong in a useful way (see OverloadResolutionResult.ReportDiagnostics)"
I'll also add one for this specific test

AlekseyTs · 2025-06-02T15:04:20Z

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

+            Diagnostic(ErrorCode.ERR_ExtensionResolutionFailed, "object.M").WithArguments("object", "M").WithLocation(4, 19));
+
+        src = """
+var x = I.M; // binds to I1.M (method)


// binds to I1.M (method)

It would be good to observe the fact. This suggestion is applicable to the other three similar comments as well. #Closed

AlekseyTs · 2025-06-02T15:04:56Z

Done with review pass (commit 1) #Closed

AlekseyTs

LGTM (commit 3). However, I personally think that the implemented rule is unnecessarily restrictive. Therefore, I suggest delaying the merge until after June 4th LDM, where I hope we finalize the design.

Revised based on LDM decision today

AlekseyTs · 2025-06-05T13:30:38Z

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

+                parameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);
+            }
+
+            foreach (var typeParameter in extension.TypeParameters)


extension.TypeParameters

Consider adding an early return if we have no type parameters #Closed

AlekseyTs · 2025-06-05T13:31:54Z

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

+            extensionParameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);
+            foreach (var parameter in parameters)
+            {
+                parameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);


parameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);

Consider breaking out early if we already recorded extension.TypeParameters.Length type parameters. #Closed

AlekseyTs · 2025-06-05T13:33:00Z

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

+            }
+
+            var usedTypeParameters = PooledHashSet<TypeParameterSymbol>.GetInstance();
+            extensionParameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);


extensionParameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);

Consider returning early if we already recorded extension.TypeParameters.Length type parameters after this. #Closed

AlekseyTs · 2025-06-05T13:34:59Z

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

+            }
+
+            var usedTypeParameters = PooledHashSet<TypeParameterSymbol>.GetInstance();
+            extensionParameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);


extensionParameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);

It feels unfortunate that we have to repeat this for every member. #Closed

Agreed. I could leave an follow-up comment in "optimization/MQ" bucket, but I think I'm fine to leave as-is

AlekseyTs · 2025-06-05T13:43:39Z

Done with review pass (commit 5) #Closed

jaredpar · 2025-06-05T17:53:43Z

@333fred second reviewer

jcouv · 2025-06-05T19:10:46Z

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


        /// <summary>
        /// For the type representing an extension declaration, returns the receiver parameter symbol.
+        /// Note: this may be null even if <see cref="IsExtension"/> is true, in error cases.


📝 I'd already added that comment to the corresponding public API, but it seems useful here too

Think you should add some remarks to indicate that it will not be null in the case of extension(SomeType), just an unnamed parameter.

AlekseyTs · 2025-06-05T19:20:00Z

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

+                    parameter.Type.VisitType(collectTypeParameters, arg: usedTypeParameters);
+                }
+
+                if (usedTypeParameters.Count == extensionArity && extensionMemberArity == 0)


if (usedTypeParameters.Count == extensionArity && extensionMemberArity == 0)

I was actually suggesting to do this inside the loop. #Closed

AlekseyTs

LGTM (commit 6), with an optimization suggestion #78758 (review)

jcouv · 2025-06-05T20:40:10Z

@333fred for second review. Thanks

333fred · 2025-06-05T20:52:16Z

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

+                return;
+            }
+
+            var usedTypeParameters = PooledHashSet<TypeParameterSymbol>.GetInstance();


Does this need some kind of comparison flags? Do we have tests where the type parameter differs by nullability? #Resolved

Does this need some kind of comparison flags? Do we have tests where the type parameter differs by nullability?

We are dealing with declarations, therefore, we don't need to worry about any substitution/alpha renaming going on.

333fred · 2025-06-05T20:59:27Z

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


        /// <summary>
        /// For the type representing an extension declaration, returns the receiver parameter symbol.
+        /// Note: this may be null even if <see cref="IsExtension"/> is true, in error cases.


Think you should add some remarks to indicate that it will not be null in the case of extension(SomeType), just an unnamed parameter.

333fred · 2025-06-05T21:01:43Z

src/Compilers/CSharp/Portable/CSharpResources.resx

  </data>
  <data name="ERR_UnderspecifiedExtension" xml:space="preserve">
-    <value>The extended type '{0}' must reference all the type parameters declared by the extension, but type parameter '{1}' is not referenced.</value>
+    <value>Every type parameter from the extension block must be referenced by the extension parameter or a parameter of this member. But type parameter `{0}` is not referenced.</value>


I think this is far too wordy.

Suggested change

<value>Every type parameter from the extension block must be referenced by the extension parameter or a parameter of this member. But type parameter `{0}` is not referenced.</value>

<value>The type parameter `{0}` is not referenced by either the extension parameter or a parameter of this member.</value>

jcouv self-assigned this May 29, 2025

jcouv added Area-Compilers Feature - Extension Everything The extension everything feature labels May 29, 2025

jcouv force-pushed the extensions-inferrability branch from 550cc8e to 8de7718 Compare May 29, 2025 21:26

Extensions: relax inferrability rule for methods

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jcouv force-pushed the extensions-inferrability branch from 8de7718 to ace0477 Compare May 29, 2025 21:47

jcouv commented May 30, 2025

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Extension type parameters must be used by parameters, except for methods

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jcouv changed the title ~~Extensions: relax inferrability rule for methods~~ Extensions: relax inferrability rule Jun 5, 2025

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	<value>Every type parameter from the extension block must be referenced by the extension parameter or a parameter of this member. But type parameter `{0}` is not referenced.</value>
	<value>The type parameter `{0}` is not referenced by either the extension parameter or a parameter of this member.</value>

Extensions: relax inferrability rule #78758

Extensions: relax inferrability rule #78758

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