Picolisp has a feature called "super parentheses":
Brackets ('[' and ']') can be used as super parentheses. A closing bracket will match [all parentheses to and including] the innermost opening bracket, or [if there are no unmatched opening brackets] all currently open parentheses. src
Taking a string containing only the characters
() (and newline if your language requires it), remove all closing brackets
 and replace them with the corresponding parentheses. A
[ is replaced with a
(, but your program will not have to handle a
( without a matching
). Output may contain trailing newlines.
] matches as many open parens as possible until it hits a
[ at which point it has to stop --DJMcMayhem
((((] -> (((()))) (([()(()])) -> (((()(())))) ) or ( or ] or [) or ([(] -> invalid, do not need to handle (] -> () ((](] -> (())()  -> () [(] -> (()) [[()(]] -> ((()())) ((][(] -> (())(()) (([((](] -> ((((()))()))
This is code-golf, shortest code per language wins.