digitalmars.D.bugs - [Issue 16104] New: Unions should allow fields with destructors,
- via Digitalmars-d-bugs (63/63) May 30 2016 https://issues.dlang.org/show_bug.cgi?id=16104
https://issues.dlang.org/show_bug.cgi?id=16104 Issue ID: 16104 Summary: Unions should allow fields with destructors, postblits, and invariants Product: D Version: D2 Hardware: x86_64 OS: Linux Status: NEW Severity: enhancement Priority: P1 Component: dmd Assignee: nobody puremagic.com Reporter: andrei erdani.com Consider https://dpaste.dzfl.pl/4897efa2f3e0, also copied here: struct A1 { this(int) immutable {} //~this(); } struct A2 { this(int) immutable {} } struct B { union { A1 a1; A2 a2; } bool itsA2; this(int x) immutable { if (x < 0) { a1 = immutable(A1)(x); } else { a2 = immutable(A2)(x); itsA2 = true; } } } Here B is able to offer an immutable constructor because the typechecker converts the first field assignment a1 = expr into a constructor call (not an assignment). Then all we need to do is provide immutable constructors for A1 and A2. Neat. Unfortunately, it all comes unglued if either or both A1 and A2 also offers a destructor. In that case they can't be members of a union any longer, and the consequence of that is, oddly, that objects with unions cannot offer immutable constructors. E.g. uncommenting the dtor in A1 causes the error: struct std.experimental.rc.B destructors, postblits and invariants are not allowed in overlapping fields a1 and a2 This is an interesting example of interacting language features. The solution is to allow literally any D type in a union. ALL TYPES. The very point of the union is "put the layout here but enforce no guarantees about the content; surrounding code will take care of that". So types with destructors, postblits, and invariants should be gleefully allowed in unions. There would be of course no calls to any of those. This is a blocker for RCStr. --
May 30 2016