[Issue 20666] New: Variant assignment from const of large type with indirections

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20666

          Issue ID: 20666
           Summary: Variant assignment from const of large type with
                    indirections
           Product: D
           Version: D2
          Hardware: All
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: simen.kjaras at gmail.com

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.

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