[Issue 11714] Improve error message for wrongly initialized thread-local class instances

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=11714



--- Comment #8 from David Nadlinger <code at klickverbot.at> 2013-12-12 05:51:44 PST ---
(In reply to comment #7)
> If I understand the logic right, the result of "new MyClass" is not knowable at
> compile time

Actually it is, otherwise you'd get a "cannot be read/evaluated at compile
time" error in the constructor.

The issue here is that in D global variables can only be initialized with
statically known data (written to the relevant sections of the object file).
For everything else, you need to use a static constructor.

While support for converting CTFE class instances into data blobs to write to
an object file was implemented a while ago, variables of a class type are of
course references and need to point somewhere. Now, if a variable is either
immutable or explicitly shared between threads, that's fine, you just emit the
data once and then initialize the variable to point to that. However, for
mutable, thread-local data, this simple scheme doesn't work, as it would
require a new instance to be created for each new thread.

I didn't really think about whether it would be possible to resolve that
situation by emitting all the instance data into the TLS section yet, but this
certainly isn't implemented in DMD right now. Thus, initializing class
references that are (potentially) modified from more than one thread with CTFE
instances is disallowed.

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