Bug 7069 – Variant Doesn't Handle Const or Immutable Contents

Example code: -------- import std.variant; class Bob {} void main() { immutable(Bob) bob = new immutable(Bob)(); Variant v = bob; immutable(Bob) bob2 = v.get!(immutable(Bob))(); } -------- Runtime Error: core.exception.AssertError@C:\D\dmd2\windows\bin\..\..\src\phobos\std\variant.d(286): immutable(Bob) This comes from here in std.variant: -------- static if (is(typeof(*cast(T*) target = *src))) { auto zat = cast(T*) target; if (src) { assert(target, "target must be non-null"); *zat = *src; } } else { // type is not assignable if (src) assert(false, A.stringof); // <-- line 286 } -------- In this case, T is of type immutable(Bob). The check for assignability fails for any immutable type (or any type that contains immutable values) even though technically this code is initializing an immutable value and should be legal. One way to make this work is to rewrite the above code to this: -------- static if (is(typeof(*cast(T*) target = *src))) { auto zat = cast(T*) target; if (src) { assert(target, "target must be non-null"); *zat = *src; } } else static if (is(T V == const(U), U) || is(T V == immutable(U), U)) { auto zat = cast(U*) target; if (src) { assert(target, "target must be non-null"); *zat = *(cast(U*) (src)); } } else { // type is not assignable if (src) assert(false, A.stringof); } -------- Which is basically casting away immutability to copy the reference. This sort of situation makes a compelling argument for the idea of a tail const reference, or a mutable reference to const or immutable data.

I've added this to my test suite for improving variant. It causes compile-time errors in my current code base, but I will work on a fix.

This bug prevents std.concurrency from working as advertised (can't send immutable messages) : http://forum.dlang.org/post/[email protected] Raising importance to "major" because of that.

This appears to have been fixed in git master: https://github.com/D-Programming-Language/phobos/commit/6e7eabbd42a7f2e3e555081e1b17893c3c18b6f8 import std.variant, std.stdio; class Bob {} void main() { immutable(Bob) bob = new immutable(Bob)(); writeln(cast(void*)bob); Variant v = bob; immutable(Bob) bob2 = v.get!(immutable(Bob))(); writeln(cast(void*)bob2); } Prints out 1091E4FF0 1091E4FF0