How to resolve ambiguous C# extension methods

Consider these two extension method signatures:

public static async Task<V> SelectMany<T, U, V>(
    this Task<T> self,
    Func<T, Task<U>> bind,
    Func<T, U, V> project) => // etc

public static Task<Validation<FAIL, C>> SelectMany<FAIL, A, B, C>(
    this Task<Validation<FAIL, A>> self,
    Func<A, Task<Validation<FAIL, B>>> bind,
    Func<A, B, C> project) => // etc

If both of these are in the same namespace then the compiler cannot always infer whether you meant Task<V> or the more specific Task<Validation<FAIL, C>> (where V is Validation<FAIL, C>).

This particular example is for extensions in the LanguageExt library, which would typically be used in a LINQ query like:

public Task<Validation<Error, int>> GetX() => // etc
public Task<Validation<Error, int>> GetY() => // etc

public Task<Validation<Error, int>> GetResult() =>
    from x from GetX()
    from y from GetY()
    select x + y;

The compiler will try to find a SelectMany implementation to bind together the rows of the LINQ query-syntax block, but, as used here, will report the choice is ambiguous.  Both extensions are in the LanguageExt namespace (though one is in LanguageExt.Core package and the other in LanguageExt.Transformers) so adding using LanguageExt brings both into scope.

One way to work around this is by using how the compiler searches for matching extension methods.

From the C# spec

The search [..] proceeds as follows:

Starting with the closest enclosing namespace declaration, continuing with each enclosing namespace declaration, and ending with the containing compilation unit, successive attempts are made to find a candidate set of extension methods:

If the given namespace or compilation unit directly contains non-generic type declarations Cᵢ with eligible extension methods Mₑ, then the set of those extension methods is the candidate set.

If namespaces imported by using namespace directives in the given namespace or compilation unit directly contain non-generic type declarations Cᵢ with eligible extension methods Mₑ, then the set of those extension methods is the candidate set.

using System;
using System.Threading.Tasks;
using LanguageExt;

namespace YourApplication;

public static class BindDisambiguationExtensions
{
    public static Task<Validation<FAIL, B>> Select<FAIL, A, B>(
        this Task<Validation<FAIL, A>>self,
        Func<A, B> f) =>
        ValidationT_AsyncSync_Extensions.Select(self, f);

    public static Task<Validation<FAIL, C>> SelectMany<FAIL, A, B, C>(
        this Task<Validation<FAIL, A>> self,
        Func<A, Task<Validation<FAIL, B>>> bind,
        Func<A, B, C> project) =>
        ValidationT_AsyncSync_Extensions.SelectMany(self, bind, project);
}