Bug 9133 – std.datetime: Cannot implicitly convert const(SysTime) to SysTime

Test case: import std.datetime; const SysTime a; SysTime b = a; Error: cannot implicitly convert expression (a) of type const(SysTime) to SysTime This is extremely surprising to me, as I would expect SysTime to have value semantics. I suppose this has to do with the timezone field? If this problem is unavoidable, I think that SysTime should either have a copy/dup method, or at the very least it should be noted in the documentation whether it is safe to cast it to mutable.

It's an issue with dmd. For whatever reason, it won't let you assign a SysTime to const or immutable even though everything in it should be convertible to const. The fact that it contains reference to a class screws it up, even though it's immutable. You have to have a postblit to fix it, but you can't have a const or immutable postblit, so you can't copy it if it's const or immutable. This problem occurs with every single struct which has reference types in it aside from dynamic arrays. It's something that needs to be solved in the language and the compiler. So, it comes down to two things: 1. dmd should be smart enough to realize that it doesn't need a postblit constructor in this case (because the class is immutable), but it isn't. 2. postblit is inherently broken with regards to const and immutable. This is a huge problem that goes way beyond just std.datetime.

I don't know when this was fixed, but it works with the current dmd master.