[Issue 7069] New: Variant Doesn't Handle Const or Immutable Contents

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7069

           Summary: Variant Doesn't Handle Const or Immutable Contents
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: debio264 at gmail.com


--- Comment #0 from Andrew Wiley <debio264 at gmail.com> 2011-12-05 21:52:07 PST ---
Example code:
--------
import std.variant;

class Bob {}

void main() {
    immutable(Bob) bob = new immutable(Bob)();
    Variant v = bob;

    immutable(Bob) bob2 = v.get!(immutable(Bob))();
}
--------
Runtime Error:
core.exception.AssertError at C:\D\dmd2\windows\bin\..\..\src\phobos\std\variant.d(286):
immutable(Bob)

This comes from here in std.variant:
--------
static if (is(typeof(*cast(T*) target = *src)))
{
    auto zat = cast(T*) target;
    if (src)
    {
        assert(target, "target must be non-null");
        *zat = *src;
    }
}
else
{
    // type is not assignable
    if (src) assert(false, A.stringof); // <-- line 286
}
--------

In this case, T is of type immutable(Bob). The check for assignability fails
for any immutable type (or any type that contains immutable values) even though
technically this code is initializing an immutable value and should be legal.

One way to make this work is to rewrite the above code to this:
--------
static if (is(typeof(*cast(T*) target = *src)))
{
    auto zat = cast(T*) target;
    if (src)
    {
        assert(target, "target must be non-null");
        *zat = *src;
    }
}
else static if (is(T V == const(U), U) || is(T V == immutable(U), U))
{
    auto zat = cast(U*) target;
    if (src)
    {
        assert(target, "target must be non-null");
        *zat = *(cast(U*) (src));
    }
}
else
{
    // type is not assignable
    if (src) assert(false, A.stringof);
}
--------

Which is basically casting away immutability to copy the reference.
This sort of situation makes a compelling argument for the idea of a tail const
reference, or a mutable reference to const or immutable data.

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