• d-bugmail puremagic.com (47/47) Aug 08 2018 https://issues.dlang.org/show_bug.cgi?id=19150

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=19150

          Issue ID: 19150
           Summary: Union member assignment causes invalid destructor call
                    in  safe code
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: snarwin+bugzilla gmail.com

The following code compiles without errors and produces an erroneous call to
HasDtor's destructor:

---
struct HasDtor
{
    int n;

    ~this()  safe
    {
        import std.stdio: writeln;
        writeln("Called destructor with n = ", n);
    }
}

union U
{
    float n;
    HasDtor h;
}

 safe void main()
{
    U u = { n : 1 };
    u.h = HasDtor(2);
}
---

Output:
Called destructor with n = 1065353216

Because there is no way to know at compile time whether the union member being
assigned to is valid, assignment to a member of a union that includes any
members with destructors should be forbidden in  safe code.

In addition, since the language must rely on the programmer to manually destroy
the appropriate union member in  system code, the automatic destructor call is
at best redundant and at worst undefined behavior. Therefore, it should be
omitted.

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