Using 2.063.2 and 2.064-devel-333c6dd: import std.concurrency : receiveOnly, send, thisTid; import std.exception : assumeUnique; import std.variant; struct JustRight { byte[32] j; } struct TooBig { byte[33] t; } void main() { JustRight[1] jArray; TooBig[1] tArray; Variant vj = jArray[]; Variant vt = tArray[]; thisTid.send(vj); thisTid.send(vt); thisTid.send(assumeUnique(jArray[])); //thisTid.send(assumeUnique(tArray[])); // Fails to compile auto received = receiveOnly!((JustRight)[]); } If the send of tArray is commented out the compilation fails with: src/phobos/std/variant.d(558): Error: cannot modify immutable expression *p So, 32-byte structs can be sent (as arrays) using send or if you first wrap it in an Variant object. Greater than 32-byte structs can be sent ONLY if they are pre-wrapped in a Variant. The size of the data shouldn't limit it from being sent using send().
Should have said: "If the send of tArray is uncommented the compilation fails with:"
There recently was a bug report for GDC/ARM regarding this. As variants max size is determined by creal which is only 16 bytes on most non-x86 architectures this error already happens with structs > 16 bytes on these architectures. Another test case with different error message: --------- struct Big { int[10] a; } import std.concurrency, std.variant, core.thread; void loop() { while(true) { Thread.sleep(seconds(10)); } } void main() { auto x = spawn(&loop); x.send(Big()); } --------- http://dpaste.dzfl.pl/f103dbac core.exception.AssertError@/opt/compilers/dmd2/include/std/variant.d(280): target must be non-null
Comment #2 is bug 9122. Comment #0 still open (the limit is 24 bytes for me).
still valid with dmd 2.068, but the location of the error changed: dmd/phobos/std/variant.d(634): Error: cannot modify immutable expression *p dmd/phobos/std/variant.d(420): Error: template instance std.variant.VariantN!32LU.VariantN.opAssign!(immutable(TooBig)) error instantiating [...]
THIS ISSUE HAS BEEN MOVED TO GITHUB https://github.com/dlang/phobos/issues/9612 DO NOT COMMENT HERE ANYMORE, NOBODY WILL SEE IT, THIS ISSUE HAS BEEN MOVED TO GITHUB