Nested function declarations

[bearophile]

This D2 code:

import std.math: sqrt;
void main() {
    double sqrt();
    double result = sqrt(9.0);
}

Generates the errors:
test.d(4): Error: function test.main.sqrt () is not callable using argument 
types (double)
test.d(4): Error: expected 0 arguments, not 1 for non-variadic function type 
double()

What is the purpose of nested function declarations in D? Is it a good idea to 
just disallow them?

Bye and thank you,
bearophile