[swift-evolution] [Idea] Allow more operators as custom operators

[Brent Royal-Gordon]

> `->`, `?` and `!` are used in types, but they are mostly unambiguous in expressions.

Sure, but types can appear in the middle of expressions. If we drop the `.self` requirement, a type name will *be* an expression. I don't think you can just ignore type names.

> The only use of `!` in expressions can be rewitten as a built-in operator function:
> postfix func ! <T> (left: T!) -> T

This doesn't work because you can assign through an `!`. For instance:

	numberDictionary[key]! += 1

Once we have inout return values, we might be able to make `!` a normal postfix operator. (Actually, the same goes for `&`, which just becomes a way to leverage the implicit `&`ing of operator arguments into an explicit `&`ing.)

> `?` is used in optional method calls:
> a.method?(b)
> A parallel proposal is going to remove such syntax from Swift, so this will not be a problem.


`?` is used for a lot more than that in expressions: optional chaining, the ternary operator, etc. None of them can be expressed as ordinary operators without significantly expanding operator syntax.

Sorry, I don't think we can make any real progress in this space without additional language features.

-- 
Brent Royal-Gordon
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