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NDLRGEN038: [RegisterClosedOverImplementationsOf] discovered type argument violates composition constraints

Cause

An implementation of the designated open generic interface was discovered whose type argument(s) do not satisfy the composition type's generic constraints. The corresponding registration is skipped and this warning is reported.

Rule Description

Unlike NDLRGEN035–NDLRGEN037, this diagnostic is emitted by the generator, not by a per-attribute analyzer, because only the generator knows the full set of discovered implementations. When a discovered type argument cannot legally close the composition, emitting new Composition<TArg>(...) would fail to compile; the generator skips that registration and reports this warning instead, turning a would-be runtime absence into a build-time signal.

This diagnostic is only reachable when the composition's constraints are stricter than the source interface's. If both carry the same constraint (e.g. where TData : class), every discovered implementation can always close the composition and this never fires.

How to Fix

Align the constraints, or prevent the incompatible implementation from being discovered:

using NexusLabs.Needlr.Generators;

// The source interface is unconstrained, so a struct argument is allowed here...
public interface IFooDefinition<TData> { }
public struct StructData { }
public sealed class StructFoo : IFooDefinition<StructData> { }

public interface IFoo { }

// ❌ WRONG - composition requires a reference type, but StructFoo closes over a struct.
//    NDLRGEN038 is reported and the StructData registration is skipped.
[RegisterClosedOverImplementationsOf(typeof(IFooDefinition<>), As = typeof(IFoo))]
public sealed class FooCore<TData> : IFoo where TData : class { }

Fixes include:

  • Constrain the source interface so incompatible implementations cannot exist (public interface IFooDefinition<TData> where TData : class).
  • Relax the composition's constraint if the composition genuinely supports the wider set.
  • Exclude the implementation if it should not participate.

See Also