[Issue 12578] Allow local function overloading

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12578

--- Comment #5 from Kenji Hara <k.hara.pg at gmail.com> ---
(In reply to Walter Bright from comment #3)
> 2. Forward references aren't allowed in local scope, meaning any use of
> overloading would be fairly restricted anyway.

By adding following rule for function local declarations, we can relax the
restriction.

- If no runtime statements exist between two local declarations,
  compiler will declare them in one DeclDef scope.

Examples:
----

void main() {
  //{
    void foo()
    void foo(int) {}
    // These are declared in one DeclDef scope, so they can be overloaded each
other.
  //}
}

The rule will work on general case.

void main() {
    struct S {
        int foo() { bar(); }
        static assert(x == 10);
    }
    void bar() { S s; }

    enum x = 10;

    // Declaraing S, bar, and x have no runtime side effect,
    // so they can be referred to each other.
}

If same name function declarations are divided to multiple DeclDefs, they
cannot be overloaded.

void main() {
    void foo()
    void foo(int) {}
    // foo == overloaded function
    // --> ok

    int x;  // runtime statement divides DeclDefs

    void foo(string) {}
    // Another declaration of foo will conflict with the overloaded foo.
    // --> error
}

--