Bug 23444 – Can't append non-copyable struct value to an array

As of DMD 2.100.2, the following program fails to compile: --- struct NoCopy { @disable this(this); } void main() { NoCopy[] a; a ~= NoCopy(); // error } --- The error message is: --- bug.d(9): Error: struct `bug.NoCopy` is not copyable because it has a disabled postblit --- However, this error is spurious. The program below compiles without errors and produces no output when run, because the postblit is not actually called at runtime: --- struct NoCopy { this(this) { import std.stdio; writeln("copied"); } } void main() { NoCopy[] a; a ~= NoCopy(); // ok; no output } ---

This seems to be done on purpose, see this comment: https://issues.dlang.org/show_bug.cgi?id=7579#c5 . In short, if you concatenate a struct instance with a disabled postblit you subject yourself to the possibility of the runtime doing copies behind your back (for example, when reallocations happen). This seems pretty nasty because the alternative would be to accept this code and then either do copies that do not call the postblit behind the scenes or have some druntime internal failures (provided that the hooks are properly templated).