Bug 11307 – Make const(T).init and immutable(T).init lvalues

".init" is a global property, that is marked as rvalue, to prevent it being modified. This is correct behavior. However, when "T" is const/immutable already, then it is illegal to modify it anyways. Making ".init" an lvalue for such cases has advantages. First, when writing "foo(const(T).init)", or "tmp = const(T).init", then pass by const reference will be preferred over pass by value. Second, a "common" use case, to memcpy initialize. This usually looks something like: T tmp = void; immutable(T) init = immutable(T).init; memcpy(&tmp, &init, T.sizeof); Making a local copy (on the stack, or in static space) is gratuitous, and can be avoided entirelly. This should work: T tmp = void; memcpy(&tmp, &immutable(T).init, T.sizeof); This code actually happens relatively often, in array and appender. So that's the ER. Nothing huge. Just keeping the T.init lvalue if T is const (or immutable). There's no reason to make it rvalue.

Does lvalueOf help with this?

(In reply to Andrej Mitrovic from comment #1) > Does lvalueOf help with this? Not really. All `lvalueOf` does is provide an "external" lvalue you can test for your traits/contraints, but it doesn't actually provide an actual instance you can read or use.

(In reply to monarchdodra from comment #0) > ".init" is a global property, that is marked as rvalue, to prevent it being > modified. This is correct behavior. > > However, when "T" is const/immutable already, then it is illegal to modify > it anyways. Making ".init" an lvalue for such cases has advantages. Not only to prevent its modification, T.init makes an rvalue to prevent copy construction on its usage. auto t = T.init; If T.init returns an lvalue, initializing t will always invoke T's postblit if exists.

(In reply to Kenji Hara from comment #3) > (In reply to monarchdodra from comment #0) > > ".init" is a global property, that is marked as rvalue, to prevent it being > > modified. This is correct behavior. > > > > However, when "T" is const/immutable already, then it is illegal to modify > > it anyways. Making ".init" an lvalue for such cases has advantages. > > Not only to prevent its modification, T.init makes an rvalue to prevent copy > construction on its usage. > > auto t = T.init; > > If T.init returns an lvalue, initializing t will always invoke T's postblit > if exists. I think that's good enough of a reason to not implement this.