Enforce static constructors lexical order in a module to respect their use order

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2014-04-27T22:04:48Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=12667

Description

This was inspired by a problem found by "spec" in D.learn:
http://forum.dlang.org/thread/gjbhmzkohtmlthnoggzr@forum.dlang.org


This reduced program:


struct Foo {
    static this() {
        Bar b;
        int x = b.data[0]; // RangeError
    }
}
struct Bar {
    static int[] data;
    static this() {
        data.length++;
    }
}
void main() {}


Gives with DMD 2.066alpha:

core.exception.RangeError@test(4): Range violation


If you swap the position of the Foo and Bar classes in the module you receive no errors.

As explained here: >Static constructors within a module are executed in the lexical order in which they appear.<

http://dlang.org/class.html#StaticConstructor

So, if Foo depends on Bar then Bar static this must appear before Foo one.

A similar example:

struct Foo {
    static this() {
        Bar b;
        int x = b.data[0];
    }
}
struct Bar {
    immutable static int[] data;
    static this() {
        data.length++;
    }
}
void main() {}


So I suggest to introduce a little D breaking change (that probably is not going to break a lot of code. In the shown case it just turns a run-time error in a compile-time one): is it possible and a good idea to raise a compilation error in such cases where code tries to use Bar static fields before bar static this() has run? (Perhaps a more fine-grained error is useful, that tracks single fields, to allow more flexible code of mutually initializing constructors, or perhaps it's not necessary).