Add optional chaining to Luau

[SeriWTR]

Summary

This RFC proposes adding an optional chaining operator (?.) to Luau to enable safe and concise access to nested table fields without requiring explicit nil checks.
Optional chaining short-circuits evaluation when the left-hand side is nil, returning nil instead of raising an error.

Motivation

Accessing deeply nested tables in Luau often requires verbose nil-checking patterns, which reduce readability and are easy to get wrong.

For example,

local languageData =
    langName
    and lang[langName]
    and lang[langName].keys

or Luau-equivalent:

local languageData =
    if langName and lang[langName] and lang[langName].keys
    then lang[langName].keys
    else nil

Design

With the optional chaining, the codes written above can be reduced to:

local languageData = lang[langName]?.keys

If lang[langName] evaluates to nil, the expression evaluates to nil without error.

Backwards Compatibility

?. operator is already invalid in Lua and Luau, making backwards-compatibility safe to implement.

Drawbacks

Optional chaining must respect existing __index and __newindex semantics.

[Kampfkarren]

FYI this has been previously approved but reverted in the past because of concern that its behavior with Roblox Instances would be non obvious.

luau-lang/luau#501

[SeriWTR]

@Kampfkarren that sucks. Because this feature would be great for non-instance objects. Because most of the time you'll still have to check typeof calls, but for tables? ?. would've been great.

[Kampfkarren]

FWIW I don't really agree with the reasoning anymore, but unless Luau team has changed their mind on this there's basically nowhere to go with it