Bug 17664 – Deprecate implicit casting between shared and unshared

--- struct A { int handle; } shared A b; void f(A a){ b=a; } --- This shouldn't compile. Value types include resource identifiers, which may have threading requirements, e.g. GUI widget handles and OpenGL handles, assuming they are thread-safe and making them implicitly shared is incorrect. union A { int handle; int* dummy; } This can't work as a workaround because it has different size than the handle.

This is not the purpose of shared. The purpose of shared is to prevent sharing memory, not other resources. Since a copy of the struct does not share any of the original memory, this needs to be allowed. You will need to implement this a different way to prevent sharing, D provides many tools for this.

The idea that sharing is opt out contradicts the idea behind shared qualifier.

Making a complete independent copy of a value is not sharing, it's copying. shared doesn't care about the semantic meaning of your integer. It only cares if multiple threads have access to the same memory in RAM. That's its only goal. Anything above that is on you to provide appropriate protections. In other words, don't use int handle, use some UnshareableResource struct that protects the handle internally from accidentally sharing.