Implement type parameter inference from constraints by agocke · Pull Request #84655 · dotnet/roslyn

agocke · 2026-07-28T05:28:48Z

Promote the generic constraints of inferred type parameters into method type inference, so that constraint-only type parameters can be inferred from the values of the type parameters they constrain (dotnet/csharplang#9453).

For example, given:

void M<TEnumerable, TElement>(TEnumerable t)
    where TEnumerable : IEnumerable<TElement>

a call M(new List<int>()) now infers TElement = int from the constraint.

The implementation extends MethodTypeInference in three ways:

Dependencies: Xi depends on Xj when Xi occurs in a constraint of Xj, so the constraining parameter is fixed first. Reflexive constraint edges are excluded.
Bound precedence: ordinary argument and output inference bounds are stored separately from constraint bounds. Constraint inference fills only parameters without ordinary bounds, preserving existing inference results.
Atomic fixing: all parameters selected by the same fixing step are determined from the same bounds state and committed together. Constraint inference occurs only after all selected parameters are fixed, making the result independent of type-parameter declaration order.

The new behavior is gated behind LanguageVersion.Preview via IDS_FeatureTypeParameterInferenceFromConstraints; existing language versions retain the original sequential path.

The dedicated TypeParameterInferenceFromConstraintsTests suite covers positive, negative, ambiguity, nullability, async-lambda/Task<T>, overload-resolution, cyclic/self-referential, order-permutation, and multi-level transitive constraint scenarios. Existing nullable baselines retain ordinary-inference behavior, while the constraint-only output-inference case now succeeds under the feature.

Specification update: dotnet/csharplang#10307

Microsoft Reviewers: Open in CodeFlow

Promote the generic constraints of inferred type parameters into method type inference, so that constraint-only type parameters can be inferred from the values of the type parameters they constrain (dotnet/csharplang#9453). For example, given: void M<TEnumerable, TElement>(TEnumerable t) where TEnumerable : IEnumerable<TElement> a call M(new List<int>()) now infers TElement = int from the constraint. Two changes to MethodTypeInference: * DependsDirectlyOn: Xi depends on Xj when Xi occurs in Xj's constraint, so a constrained parameter is fixed before the parameters mentioned in its constraint. * Fix: after a type parameter is fixed to V, perform a lower-bound inference from V into each of its constraint types, seeding bounds for the (later-fixed) parameters that appear there. Both are gated behind LanguageVersion.Preview via the new IDS_FeatureTypeParameterInferenceFromConstraints feature id. Adds a dedicated test suite (TypeParameterInferenceFromConstraintsTests) covering positive, negative, ambiguity, nullable, async-lambda/Task<T>, overload-resolution, cyclic/self-referential, and multi-level transitive constraint scenarios, and updates 9 existing nullable tests whose inferred results legitimately improve under the feature. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

azure-pipelines · 2026-07-28T05:29:07Z

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Copilot

Pull request overview

This PR extends C# method type inference (preview-only) to allow inferring type parameters that appear only in generic constraints, and adds/updates compiler tests and feature-localization resources to support the new behavior.

Changes:

Update MethodTypeInference to (1) incorporate constraint-based dependencies and (2) seed inference bounds from fixed type parameters into their constraint types (gated by a new preview feature ID).
Add a new dedicated test suite for “type parameter inference from constraints” and update existing nullable reference type tests whose inference results change under preview.
Add a new compiler feature string (IDS_FeatureTypeParameterInferenceFromConstraints) and propagate it to localized .xlf files.

Show a summary per file

File	Description
src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs	Implements preview-gated constraint-based dependency ordering and constraint-driven lower-bound seeding during fixing.
src/Compilers/CSharp/Portable/Errors/MessageID.cs	Adds a new preview feature ID for gating the behavior.
src/Compilers/CSharp/Portable/CSharpResources.resx	Adds the feature string for diagnostics/feature gating text.
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf	Adds localized XLF entry for the new feature string.
src/Compilers/CSharp/Test/Semantic/Semantics/TypeParameterInferenceFromConstraintsTests.cs	New test suite covering positive/negative and edge cases for the new inference behavior.
src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs	Updates expected diagnostics where preview inference results (and downstream nullability diagnostics) change.

Copilot's findings

Files reviewed: 18/18 changed files
Comments generated: 1

jjonescz · 2026-08-07T09:10:35Z

            comp1.VerifyDiagnostics(
-                // (9,9): warning CS8631: The type 'TB1?' cannot be used as type parameter 'TM2' in the generic type or method 'B<TB1>.M1<TM1, TM2>(TM1, TM2)'. Nullability of type argument 'TB1?' doesn't match constraint type 'TB1'.
-                //         M1(b2, a2); // 1
-                Diagnostic(ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint, "M1").WithArguments("B<TB1>.M1<TM1, TM2>(TM1, TM2)", "TB1", "TM2", "TB1?").WithLocation(9, 9),


Consider also verifying the original diagnostics under LangVersion=14

jjonescz · 2026-08-07T09:14:35Z

+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular14).VerifyDiagnostics(


For a langversion-focused test, consider testing the same snippet under Regular14, RegularNext and RegularPreview.

Regular15 now that that work has merged.

This test can probably be combined with Basic_IEnumerable under the different langversions.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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Copilot's findings

Files reviewed: 18/18 changed files
Comments generated: 0 new

Co-authored-by: Andy Gocke <angocke@microsoft.com>

Copilot

Copilot's findings

Files reviewed: 18/18 changed files
Comments generated: 0 new

Fix preview constraint inference in simultaneous waves while preserving ordinary argument and output inference bounds. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1fcf8092-6a5f-485e-891d-26fe95919661

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Review details

Files reviewed: 18/18 changed files
Comments generated: 0 new
Review effort level: Lite

333fred · 2026-08-12T21:40:23Z

Test plan: #84868
(created by new-compiler-feature skill)

Use the specification terms ordinary bounds, effective bounds, and selected fixing parameters throughout the implementation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1fcf8092-6a5f-485e-891d-26fe95919661

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Review effort level: Lite

333fred · 2026-08-17T21:30:34Z

        private readonly HashSet<TypeWithAnnotations>[] _upperBounds;
        private readonly HashSet<TypeWithAnnotations>[] _lowerBounds;

+        // Constraint-derived bounds are used only when the corresponding type parameter


Consider nullable-enabling these.

333fred · 2026-08-17T22:30:10Z

+
+            if (IsFeatureTypeParameterInferenceFromConstraintsEnabled)
+            {
+                if (!FixSelectedParameters(needsFixing, ref useSiteInfo))


It's unclear from this code what the actual difference is here; I think a combination of more clear method names and some comments would help clarify why the new path calls a method that appears, at first read, like it should handle both old and new paths.

333fred · 2026-08-17T22:38:07Z

+                    }
+
+                    Debug.Assert(IsUnfixed(iParam));
+                    if (!HasEffectiveBound(iParam) || !FixParameter(iParam, ref useSiteInfo))


Shouldn't this be HasOrdinaryBound, to match prior language version code?

333fred · 2026-08-17T22:49:48Z

+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular14).VerifyDiagnostics(


Regular15 now that that work has merged.

333fred · 2026-08-17T22:50:23Z

+                }
+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.Regular14).VerifyDiagnostics(


This test can probably be combined with Basic_IEnumerable under the different langversions.

333fred · 2026-08-17T22:58:20Z

+                """;
+
+            CreateCompilation(source, parseOptions: TestOptions.RegularPreview).VerifyDiagnostics(
+                // (11,9): error CS0311: The type 'System.Collections.Generic.List<int>' cannot be used as type parameter 'TEnumerable' in the generic type or method 'C.M<TEnumerable, TElement>(TEnumerable, TElement)'. There is no implicit reference conversion from 'System.Collections.Generic.List<int>' to 'System.Collections.Generic.IEnumerable<string>'.


This isn't a particularly good error, and definitely not what I would have expected out of fixing. We know from ordinary bounds that TEnumerable must be List<int>; the issue is in TElement. Why would we then be erroring on TEnumerable?

…ion/MethodTypeInference.cs Co-authored-by: Fred Silberberg <fred@silberberg.xyz>

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Review details

Files reviewed: 18/18 changed files
Comments generated: 0 new
Review effort level: Lite

Copilot

Review details

Files reviewed: 18/18 changed files
Comments generated: 1
Review effort level: Lite


                // C# preview features.
                case MessageID.IDS_FeatureUnsafeEvolution: // https://github.com/dotnet/roslyn/issues/82546: keep this in preview until C# 16
+                case MessageID.IDS_FeatureTypeParameterInferenceFromConstraints:    


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Files reviewed: 32/32 changed files
Comments generated: 0 new
Review effort level: Lite

333fred

Looks like a few comments from previous review are still unaddressed.

333fred · 2026-08-20T22:40:22Z

+  the disabled-feature boundary; retain an explicit `RegularPreview` case when
+  testing the feature's language-version gate.


retain an explicit RegularPreview case when testing the feature's language-version gate.

This guidance seems incorrect? To test the gate, you want to test across N and RegularNext.

Copilot AI lite review requested due to automatic review settings July 28, 2026 05:28

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Comment thread src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/MethodTypeInference.cs

build-analysis Bot mentioned this pull request Jul 28, 2026

[Known Build Error] Test_Windows_Desktop_Release_32: Test host process crashed #84579

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agocke requested a review from 333fred July 28, 2026 17:42

build-analysis Bot mentioned this pull request Jul 28, 2026

DaemonServerLifecycleTests.DaemonKilled_EveryThinClientExitsNonZero fails intermittently #84662

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agocke mentioned this pull request Jul 28, 2026

[Proposal]: Type Parameter Inference from Constraints dotnet/csharplang#9453

Open

jjonescz reviewed Aug 7, 2026

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Potential fix for pull request finding

80f7ea4

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Apply batched suggestions from code review

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Co-authored-by: Andy Gocke <angocke@microsoft.com>

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Make constraint inference order-independent

5cde083

Fix preview constraint inference in simultaneous waves while preserving ordinary argument and output inference bounds. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1fcf8092-6a5f-485e-891d-26fe95919661

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333fred added the Feature - Type Inference from Constraints label Aug 12, 2026

333fred added the Area-Compilers label Aug 12, 2026

agocke mentioned this pull request Aug 14, 2026

Update type parameter inference from constraints specification dotnet/csharplang#10307

Open

Align constraint inference terminology

1894c2c

Use the specification terms ordinary bounds, effective bounds, and selected fixing parameters throughout the implementation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 1fcf8092-6a5f-485e-891d-26fe95919661

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Update src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolut…

486ddea

…ion/MethodTypeInference.cs Co-authored-by: Fred Silberberg <fred@silberberg.xyz>

Copilot AI review requested due to automatic review settings August 19, 2026 17:39

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Merge branch 'main' into infer-generic-constraints

3d83839

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Comment thread src/Compilers/CSharp/Portable/Errors/MessageID.cs Outdated

// C# preview features.

case MessageID.IDS_FeatureUnsafeEvolution: // https://github.com/dotnet/roslyn/issues/82546: keep this in preview until C# 16

case MessageID.IDS_FeatureTypeParameterInferenceFromConstraints:

agocke added 4 commits August 19, 2026 18:42

Move constraint inference tests to CSharp16

c17be47

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 617fedad-8306-4126-b532-e273eecf2248

Document constraint inference breaking change

59eb275

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 617fedad-8306-4126-b532-e273eecf2248

Remove trailing whitespace from feature gate

4212a5e

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Fix copilot mistakes

40476de

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		the disabled-feature boundary; retain an explicit `RegularPreview` case when
		testing the feature's language-version gate.

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agocke commented Jul 28, 2026 •

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