Allow out, in, and ref in generic type parameters.

[colejohnson66]

Currently, generics don't support out, in, or ref on type parameters. This causes issues when one wants to use Action and Func delegates. For example, Func<ref int, int> does not work. Instead, the programmer is forced to create their own delegate type:

public delegate TOutput RefFunc<TInput, TOutput>(ref TInput param);

I propose that this limitation be lifted so that one could write Func<ref int, int> without error. Normal generic methods would work the same way:

int x = 0;
Test<ref int>(ref x);

void Test<T>(T param) { }

The compiler would "fill in" the type parameters into the definition, and complain if something doesn't make sense. For example, the following would cause a compiler error:

Func<out int> x; // error about usage of an "out" parameter in return position
Test<in int>();  // error about usage of an "in" parameter in return position

T Test<T>() => default(T)!;

[HaloFour]

You can just define a different delegate type to support ref or out parameters. The runtime forbids the use of ref types for generic parameters as the IL required to work with them are necessarily different.

[siegfriedpammer]

There is a proposal for this already #5321

[CyrusNajmabadi]

We'd likely do this, if the runtime adds support for it.

[HaloFour]

From the proposal it sounds like it would be limited to delegates, and only where the use is defined as ref, so you couldn't use ref T for M<T>(Action<T>), but you could use M<T>(Action<ref T>). Sounds like a lot of hoops for very little benefit.