[Issue 21665] New: Void initialization should not be allowed for instances of struct with invariant

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21665

          Issue ID: 21665
           Summary: Void initialization should not be allowed for
                    instances of struct with invariant
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

struct ShortString {
    private ubyte length;
    private char[15] data;

    @safe
    this(const(char)[] src) {
        length = cast(ubyte) src.length;
        data[0 .. src.length] = src[];
    }

    @trusted
    const(char)[] opIndex() const {
        // should be ok to skip the bounds check here
        return data.ptr[0 .. length];
    }

    invariant { assert(length <= data.length); }
  }

If this is void initialized,

  @safe test() {
    ShortString s = void;
    char c = s[16];
  }

Then the value in .length is unpredictable, so the invariant may not be
satisfied. So, in @safe code, void initializations of struct objects when the
struct has an invariant should not be allowed.

The same holds true for unions, such as:

  union U {
    int n;
    ShortString s;
  }

Examples by Dennis Korpel and Paul Backus.

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