Bug 20117 – std.typecons.Typedef has opCmp when base type does not

---------------------------------- import std.traits; import std.typecons; alias MyInt = Typedef!int; pragma(msg, hasMember!(int, "opCmp")); pragma(msg, hasMember!(MyInt, "opCmp")); ---------------------------------- This prints false then true. It doesn't work very well in generic code.

This would perhaps more correctly be filed as 'int does not have opCmp'. Since int does support the <, <=, >=, > operators it kinda, sorta has opCmp, but no member by that name. That's not really an option for Typedef, as it's not a magic type like int. At the same time, we absolutely expect MyInt.init < MyInt.init to compile. There may be a good argument for adding opCmp and friends to built-in types to make generic code more generic, but removing them from Typedef is no solution.

Alternatively, Typedef should contain only a single member and use aliases for the rest. Something like this: struct Typedef(T, T init = T.init, string cookie=null) { // Renaming the current Typedef: TypedefImpl!(T, init, cookie) _payload; alias _payload this; static foreach (e; __traits(allMembers, T)) mixin("alias "~e~" = _payload."~e~";"); }

But `int` *doesn't* have `opCmp`. I found this by trying to wrap a generic type for Python and implementing the comparison. std.typecons.Typedef completely broke things and I have to special-case for it, because `hasMember!(Typedef, "opCmp")` returns true, but then one can't actually forward to it because it doesn't actually exist. Incidentally, the reason the bug happens is because `Typedef` uses `Proxy` in its implementation, and `Proxy` defines `opCmp`.

But `int` *is* comparable using <, <=, >=, and >, so any (non-built-in) type pretending to be an int needs to have `opCmp`. What you're asking for is basically to cripple `Typedef` because you need to write some workaround code. Alternatively, you're asking for what I said in comment 2, but have elected to ignore that. Actually, even with my `alias this` solution in comment 2, you get two members: `_payload`, and `opAssign`. The former we've defined, the latter the compiler makes for us. `int` does not have `opAssign`, so we can't forward to that, thus again causing possible issues. In fact, `int` doesn't have any members at all, if you ask `__traits(allMembers)`. Thus, any member whatsoever in `Typedef!int` could be problematic in generic code. I hope we can agree that `Typedef!int` needs to have at least one member. Actually, we can get rid of all fields by (ab)using `align` and some creative casting: template Typedef(T) { align ((T.sizeof + 1) & -2) // powers of 2 only struct Typedef { ref auto _payload() { import std.typecons : TypedefImpl = Typedef; return *cast(TypedefImpl!T*)&this; } alias _payload this; // alias all members of the wrapped type static if (__traits(compiles, __traits(allMembers, T))) static foreach (e; __traits(allMembers, T)) mixin("alias _payload."~e~" "~e~";"); } } This still leaves us with a single method (`_payload()`) that I haven't found a way to get rid of yet. Also, it's hella ugly.

THIS ISSUE HAS BEEN MOVED TO GITHUB https://github.com/dlang/phobos/issues/10382 DO NOT COMMENT HERE ANYMORE, NOBODY WILL SEE IT, THIS ISSUE HAS BEEN MOVED TO GITHUB