[Proposal]: Type Parameter Inference from Constraints

[333fred]

Type Parameter Inference from Constraints

• Specification: Infer type parameters from constraints #9455
• Discussion: Type Parameter Inference from Constraints #9454
• Implementation: Implement type parameter inference from constraints roslyn#84655

Summary

Allow type inference to succeed in overload resolution scenarios by promoting generic constraints of inferred type parameters to "fake arguments" during type inference, enabling
the bounds of type variables to participate in the inference process. An example is:

List<int> l = [1, 2, 3];
M(l); // Today: TElement cannot be inferred. With this proposal, successful call.

void M<TEnumerable, TElement>(TEnumerable t) where TEnumerable : IEnumerable<TElement>
{
    Console.WriteLine(string.Join(",", t));
}

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-30.md#type-parameter-inference-from-constraints
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-18.md#type-inference
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-08-12.md#type-parameter-inference-from-constraints-implementation-review