When a type T is small enough, Variant allows assignment from it even if it is not implicitly convertible to Unqual!T. However, when the size of the type is larger than VariantN's size argument, a different path is chosen, and the compiler chokes. This example code showcases the effect - for small i there is no problem, but then there's an compile error when i > 16, 12, or 8 (depending on compiler and flags):
struct S(int padding) {
byte[padding] _;
int* p;
}
unittest {
import std.variant;
static foreach (i; 0..64) {{
const S!i s;
Variant a = s;
}}
}
The code on which the compiler chokes is this:
https://github.com/dlang/phobos/blob/master/std/variant.d#L699-L701
I'm unsure why larger types are treated differently. The comment on lines 667-674 seems to indicate it has to do with whether they're passed on the stack or not, but this assumption does not seem to be documented elsewhere.
Comment #1 by robert.schadek — 2024-12-01T16:36:31Z