On 4/1/22 6:22 PM, Vijay Nayar wrote:
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
```
The issue is that for some reason redblacktree declares a `@safe nothrow
toHash` function.
https://github.com/dlang/phobos/blob/99e9c1b7741e0f4e6f2a8c14883c4828d092701d/std/container/rbtree.d#L1113
I'm not sure why that is. But just copying a Variant is not `@safe
nothrow`. So this explains why it's not working.
RBT should detect whether it can call the toHash with the appropriate
attributes and declare it based on that.
-Steve
