GNU C has had nested functions for quite a long time. I would use them a lot if ...

WalterBright · on May 26, 2020

D's differs in that:

1. you cannot "goto" out of the nested function

2. you can declare them "static", which means they don't have a static link and cannot access the enclosing function's variables

3. if garbage collection is enabled, you can pass a pointer to the function out of the enclosing function's scope and still safely access its variables

4. they work in `extern (C++)` functions, too