Consider the following program:
```d
void main()
{
import std.stdio;
import std.container.rbtree;
import std.variant;
// alias Type = int; // Works with no problem.
alias Type = Variant; // Produces error.
auto rbTree = new RedBlackTree!(Type);
rbTree.stableInsert(Type(3));
rbTree.stableInsert(Type(2));
rbTree.stableInsert(Type(4));
foreach (v; rbTree.upperBound(Type(2))) {
writeln(v);
}
}
```
A `RedBlackTree` constructs and runs perfectly fine using "int"
as the data type, but it seems to blow up as soon as I use
`std.variant : Variant`.
```
Compilation output (1: )
/dlang/dmd/linux/bin64/../../src/phobos/std/container/rbtree.d(1116): Error: `@safe`
function `std.container.rbtree.RedBlackTree!(VariantN!32LU, "a < b",
false).RedBlackTree.toHash` cannot call `@system` function
`std.container.rbtree.RBRange!(RBNode!(VariantN!32LU)*).RBRange.front`
/dlang/dmd/linux/bin64/../../src/phobos/std/container/rbtree.d(682):
`std.container.rbtree.RBRange!(RBNode!(VariantN!32LU)*).RBRange.front` is
declared here
/dlang/dmd/linux/bin64/../../src/phobos/std/container/rbtree.d(1116): Error:
destructor `std.variant.VariantN!32LU.VariantN.~this` is not `nothrow`
/dlang/dmd/linux/bin64/../../src/phobos/std/container/rbtree.d(1113): Error: function
`std.container.rbtree.RedBlackTree!(VariantN!32LU, "a < b",
false).RedBlackTree.toHash` may throw but is marked as `nothrow`
onlineapp.d(10): Error: template instance
`std.container.rbtree.RedBlackTree!(VariantN!32LU, "a < b",
false)` error instantiating
```
This may seem like a strange thing to do, but it is useful when
trying to have set of indexes, each of which is used on a
different data field. The set of indexes can share a common
data-type using Variant, which allows one to do things like have
associative arrays of indexes which you can look up by field name.
However, it looks like `Variant` doesn't have the various
attributes desired by `RedBlackTree`. Is there a way to make them
compatible? Do I need to avoid std.container.rbtree and make my
own which doesn't require these attributes, or do I need to
implement my own version of Variant using a union and try to make
it safe/nothrow/etc.?
