?-> can be used to index something that is either indexable or null.
?: is an alias for ||

Very useful for things like:

C c = a && a->b && a->b->c;

which can now be rewritten as

C c = a?->b?->c;

The code in language.yacc also supports ?. and ?[], but there are
syntax conflicts with ?: for those:

?. conflicts with expr?.Module:...
?[] conflicts wtih expr?[softcast]:...

?: was added mostly to be compatible with other 'modern' C-like
languages.