Fix Nullalble to throw logic errors instead of exceptions

[monarchdodra]

See also:
http://d.puremagic.com/issues/show_bug.cgi?id=9404

Basically, Nullable[Flavor] was not self assignable. Attempts to do self assignement would compile, because it extracts the value value via get, and then uses opAssign(T value). This can throw an exception if the other nullable is null though.

The problem with this is that it makes this(this) andopAssign` inconsistent (postblit never throws). This makes things particularly tricky if you start nesting a Nullable inside a struct:

struct S1
{
    Nullable!int ni;
    //default S1 s1 = S1(myNullable); works
    //default s1 = s2; works
}

struct S2
{
    Nullable!int ni;
    this(Nullable!int n)
    {ni = n;} //Throws
    void opAssign(S2 other)
    {ni = other.ni;} //Throws
}

---

This pull simply give all Nullables the function void opAssign()(typeof(this) other). This makes the current nullable a copy of other.

Also in this pull:

• Remove 4424 workaround
• Give failed enforcements error messages
• Workaround for 9416
• Extra unittests

[monarchdodra]

While I realize the if is not strictly needed, and that objects can be legally destroyed multiple times (which is what happens anyways when the nullable itself is destroyed), I figure we can still avoid doing so when possible.

[monarchdodra]

I also have a question/comment, that's related (though won't change the current pull):

Nullable.get will throw an exception if you read it while it is null. Afaik, a null read has always been an error.

Making an exception means

• Code can't be nothrow
• Extra costs when you know your nullable isn't null anyways.

Any chance we can make "get" throw errors? Or is that a breaking change?

[alexrp]

It would be a breaking change, but I highly doubt anyone actually would be affected by it. Getting the value of a null nullable is a logic error for sure.

[monarchdodra]

OK. Thanks. I'll save the change for a subsequent pull.

[monarchdodra]

Given that Kenji correct the initial 9404 problem, my fix is not needed anymore.

I'm preserving the unittests and improvements though, and doing the enforce => assert change.