Which operators cannot be overloaded and why not?

[jfondren]

On Monday, 13 September 2021 at 14:33:03 UTC, user1234 wrote:
> - condition al expression ` cond ? exp : exp `

And many other boolean operators, unary !, binary && and ||

https://dlang.org/spec/operatoroverloading.html lists all the 
overloadable operators, and 
https://dlang.org/spec/expression.html has all the operators, so 
"which operators" is a matter of close comparison. "why not" is 
much harder to answer.