[Proposal]: Type inference using method group natural type

[jcouv]

Type inference using method group natural type

Champion issue: #9007

Summary

It allows the natural type of a method group to contribute to method type inference

Context on method type inference

The current type inference rules allow

• method groups to contribute to output type inference
• infering bounds for the Ui type parameters in C<U1 ... Uk> when given a C<V1...Vk> and C is a class, struct, interface or delegate type.
  But they don't allow method groups to contribute to bounds.

Test(IsEven); // Error CS0411	The type arguments for method 'Program.Test<T>(Func<T, bool>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

partial class Program
{
    public static bool IsEven(int x) => x % 2 == 0;
    public static void Test<T>(Func<T, bool> predicate) { }
}

Type inference

We modify the explicit parameter type inference rules to not just apply to explicitly-typed lambdas, but also to method groups:

An explicit parameter type inference is made from an expression E to a type T in the following way:

• If E is an explicitly typed anonymous function *or method group with a unique signature with parameter types U₁...Uᵥ and T is a delegate type or expression tree type with parameter types V₁...Vᵥ then for each Uᵢ an exact inference is made from Uᵢ to the corresponding Vᵢ.

---

Follow-ups:

• we need to confirm this was not left out intentionally from the "lambda improvements" work
• outline and review any potential breaking changes

Relates to issue for method groups contributing to method type inference: #129

LDM Discussions

• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-04.md#type-inference-using-method-group-natural-type