Forward declaration issue

[Jonathan M Davis via Digitalmars-d-learn]

On Friday, December 04, 2015 08:12:05 Andre via Digitalmars-d-learn wrote:
> Hi,
>
> I have a strange issue with following coding.
>
> void baz(); // forward declaration
>
> void foo()
> {
>   void bar()
>   {
>       baz(); // (1) without f.d. syntax error
>   }
>
>   void baz()
>   {
>       bar();
>   }
>
>   baz(); // (2) No linker error if line is removed
> }
>
> void main()
> {
>   foo();
> }
>
> Without the forward declaration, there is a syntax error at (1)
> With the forward declaration there is no syntax error but
> a linker error at (2). This linker error disappears if line at (2)
> is removed.
> It looks like a bug, is it?

You cannot use symbols before you declare them in a function (even if
they're nested functions), and you can't forward declare them. When you
declare baz outside of foo, bar is now trying to use a different baz from
the one that you declare after it. Rather, it's trying to use one that's at
the module-level, not a nested function. And you never defined that baz. So,
you get a linker error when you use it. What's going on would be clearer if
you used distinct names:


void module_baz();

void foo()
{
    void bar() { module_baz(); }
    void baz() { bar(); }

    baz();
}


While that may not be what you're trying to do, it's what you're actually
doing. Mutually recursive nested functions aren't possible in D.

- Jonathan M Davis