Proposal: Infer generic arguments type by pattern matching on where constructs

[tmm360]

I'm not sure that this had not been discussed, but from a rapid search I didn't find this proposal.
I've code like this:

class ModelBase<TKey>
{
    TKey Id { get; set; }
}
class Repo<TModel, TKey> :
    where TModel : ModelBase<TKey>
{
    //code where I access both TModel and TKey types
}

In my Repo class, I need to access both the TModel and TKey, but when I specific the TModel argument, a pattern matching system should be able to extract the TKey automatically.

For use the TKey generic type, I have to declare it as a generic argument, so every time that I need to create a Repo, I have to specific model and relative key types.

My propose is to implement generic type inference from where constructs, so TKey could be removed from class declaration, and Repo could be written like this:

class Repo<TModel> :
    where TModel : ModelBase<TKey>
{
    //code where I access both TModel and TKey types
}

[HaloFour]

The problem with that approach is that the CLR treats generic types as unique based on their arity of generic type parameters, so Repo<TModel> and Repo<TModel, TKey> are technically two distinct types. To the CLR (and any other languauge), this type would still have to be Repo<TModel, TKey>.

[tmm360]

And if Repo class declaration would be something like this?

class Repo<TModel, implicit TKey> :
    where TModel : ModelBase<TKey>
{
    //code where I access both TModel and TKey types
}

This could say to CLR that type is a Repo<TModel, TKey>, but it doesn't force you to explicit that. Obviously, if I declare also a Repo<TClass> class an ambiguous warning should be raised.

For avoid ambiguity, I could also choose a keyword for autocomplete the field, or simply leave it empty like this:

class MyModel : ModelBase<string>
{ }

var repo = new Repo<MyModel, >(); //repo is Repo<MyModel, string>