Bug 20728 – There is third way to create immutable data

https://dlang.org/spec/const3.html says that there are two ways of "Creating immutable data". I disagree, there is also the third way: auto int x = 1; void f(ref immutable int r) { } f(x); This is neither literal nor cast to immutable data. It is the third way. See also https://forum.dlang.org/post/[email protected] as a hint about the issue whether it actually creates immutable data.

Your sample code doesn't even compile. That's because of this rule. That page doesn't mention the implicit cast of unique data (such as returned from a pure function), though that is technically a cast so it is kinda covered. But regardless, your example is invalid so I'm closing this. Please make sure your code actually does what you think it does before opening a bug.