• d-bugmail puremagic.com (48/52) Mar 07 2018 https://issues.dlang.org/show_bug.cgi?id=18568

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=18568

          Issue ID: 18568
           Summary: partially overlapping assignments have undefined
                    behavior but are accepted in  safe code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: safe
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: ag0aep6g gmail.com

Prompted by this forum post:
https://forum.dlang.org/post/kslpmklgrgwaynlbkrph forum.dlang.org

On assignments, the spec says [1]:

 Undefined Behavior:
 1. if the lvalue and rvalue have partially overlapping storage
 2. if the lvalue and rvalue's storage overlaps exactly but the types are
    different

But DMD accepts this:

----
struct S
{
    union
    {
        int i;
        byte b;
        float f;
        struct
        {
            byte b2;
            align(1) int i2;
        }
    }
}

void main()  safe
{
    S s;
    s.i = s.b; /* Partially overlapping, different types. */
    s.f = s.i; /* Exactly overlapping, different types. */
    s.i = s.i2; /* Partially overlapping, same type. */
}
----

According to the spec, all those assignments have undefined behavior. So they
shouldn't be allowed in  safe code.

(As always, this can be fixed by letting DMD reject the code, or by changing
the spec to give the code defined behavior.)


[1] https://dlang.org/spec/expression.html#assign_expressions

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