Issue Description:

I was playing around with @dynamicMemberLookup, trying to implement a library version of something like C#'s built-in dynamic feature.

@dynamicMemberLookup
class Dynamic {
    private var members = [String: Any]()
    
    subscript<T>(dynamicMember member: String) -> T! {
        get { members[member] as? T }
        set { members[member] = newValue }
    }
}

let x = Dynamic()
x.i = 1
x.s = "abc"
x.f = { $0 + 10 } as (Int) -> Int

print(x.i as Int)
print(x.s as String)
let i: Int = x.f(10) # Generic parameter 'T' could not be inferred
print(i)

It's a silly example, and I can't imagine myself using something like this in prod, but it seems like it should be possible, anyway.

The expression x.f(10) should have enough information for the generic return type to be inferred.

• It's known to be a function, because it's being called with ()

• It's known to return an Int.

• It's known to have one parameter, which is an integer literal, which in the absence of other constraints should be inferred to IntegerLiteralType (Int by default)