Allow "eliding" the typing of all generic arguments

[colejohnson66]

Generic argument inference is great, but it still has some sharp edges. Namely, the ability to elide specification of the entire generic qualifier if (even just) one type is not in the parameter list.

For the current state of C#, say I have some function that takes T and gives back T. In that situation, I can elide the <T> argument on the function call. However, if I have a function that takes TInput and gives back TOutput, I'm forced to qualify the entirety of <TInput, TOutput> on the call.

// function contents are irrelevant
public T OneArity<T>(T input) => ...;
public TOutput TwoArity<TInput>(TInput input) => ...;

// can elide the one-arity call, but can't for the two-arity call
int one = OneArity(42); // or OneArity<int>(42)
int two = TwoArity<double, int>(42.0);

Proposal

I propose the "sink" keyword _ be allowed as a type specifier in generics. Using the "sink specifier" would instruct the compiler to infer the type, just as it does already, but for only a portion of the specifier. In such a situation, one would be able to write the following:

int one = OneArity(42); // or OneArity<_>(42)
int two = TwoArity<_, int>(42.0);

Specification Change

The C# specification (section 8.4.2) would be changed as follows:

 type_argument_list
     : '<' type_arguments '>'
     ;

 type_arguments
     : type_argument (',' type_argument)*
     ;   

 type_argument
     : type
+    : '_'
     ;

Backwards Compatibility

The only possible hiccup this proposal could cause is for users who have a type called _. However, users generally don't do that. Regardless, the remedy to a conflict would be to consider _ as an elision if, and only if, no type named _ exists in the current scope.

[333fred]

Duplicate of #1349, which @TomatorCZ is actively working on specing in #7582.