Should we provide a syntactic shorthand for `Core.Optional`?

[zygoloid]

Summary of issue:

Should we have a syntactic shorthand for Core.Optional(T)? This type will come up frequently in interop scenarios, as C++'s T* is expected to map into Core.Optional(T*) in Carbon.

Details:

We brainstormed on this during the 2025-07 Carbon summit, but didn't come to a definitive conclusion.

Leading options seem to be:

• Postfix ?, matching postfix * for pointers, so C++ T* becomes Carbon T*?. This then needs a solution that's not postfix ? for a "try" operator.
• Prefix ?, leaving postfix ? for a "try" operator. If we do this, we would likely want to make * for pointers prefix instead.
• A keyword, perhaps opt, either prefix or postfix, perhaps with parens: opt T* or T* opt or opt(T*).

Any other information that you want to share?

For a "try" operator, we've also been considering postfix .try, which would leave syntactic space for postfix ? as a type operator.

Switching to prefix * and ? for types, and postfix * and ? for expressions, would allow us to provide a more consistent operator set with fewer parentheses required, by making ~all of our value-level unary operators postfix. For example, p->x becomes p*.x, and *(*p)->y becomes p**.y*. But this would cost us a lot of our C++ familiarity budget, probably more than we have for Carbon in total.