[Issue 24865] New: __traits(hasMember, T, "__xdtor") is true in some cases where there is no __xdtor member

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24865

          Issue ID: 24865
           Summary: __traits(hasMember, T, "__xdtor") is true in some
                    cases where there is no __xdtor member
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: issues.dlang at jmdavisProg.com

This code

---
void main()
{
    import std.stdio;
    import std.traits;

    static struct S2 { ~this() { writeln("destroyed"); } }
    static struct S3 { S2 field; }
    static struct S6 { S3[0] field; }

    S6 s6;

    pragma(msg, "S2: " ~ __traits(allMembers, S2).stringof);
    pragma(msg, "S2: " ~ __traits(hasMember, S2, "__xdtor").stringof);

    pragma(msg, "S3: " ~ __traits(allMembers, S3).stringof);
    pragma(msg, "S3: " ~ __traits(hasMember, S3, "__xdtor").stringof);

    pragma(msg, "S6: " ~ __traits(allMembers, S6).stringof);
    pragma(msg, "S6: " ~ __traits(hasMember, S6, "__xdtor").stringof);
}
---

prints out

---
S2: AliasSeq!("__dtor", "__xdtor", "opAssign")
S2: true
S3: AliasSeq!("field", "__xdtor", "opAssign")
S3: true
S6: AliasSeq!("field", "opAssign")
S6: true
---

when compiling, and it prints out nothing when it runs.

S6 should not have a destructor, because while the elements of its static array
have destructors, the static array has no length, and so, there's nothing to
destroy. The fact that nothing prints out when running the program shows that
S6 has no destructor. allMembers also has no __xdtor member, which lines up
with the behavior of the running program. However, for some reason, hasMember
reports that there _is_ an __xdtor member.

Given that allMembers matches the actual behavior, I'm inclined to think that
the bug is in hasMember rather than there being an invisible __xdtor that
allMembers isn't reporting, but either way, there is a discrepancy here which
needs to be fixed.

This affects the implementation of
{core.internal,std}.traits.hasElaborateDestructor.

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