Aliases to mutable thread-local data not allowed

[mark]

I just tried:

             auto jobs = tids.length;
             while (jobs) {
                 receive(
                     (Deb deb) { debForName[deb.name] = 
cast(Deb)deb; },
                     (DoneMessage m) { jobs--; }
                 );
             }

to cast away the immutable from the sender, but that doesn't work 
either:

src/model.d(85,30): Error: template std.concurrency.spawn cannot 
deduce function from argument types !()(void delegate(Tid 
parentTid, string filename), Tid, string), candidates are:
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/concurrency.d(460,5):        spawn(F, T...)(F fn, T args)
   with F = void delegate(Tid, string),
        T = (Tid, string)
   must satisfy the following constraint:
        isSpawnable!(F, T)
src/model.d(86,13): Warning: statement is not reachable
src/model.d(87,13): Warning: statement is not reachable
src/model.d(86,13): Warning: statement is not reachable
src/model.d(87,13): Warning: statement is not reachable
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/variant.d(701,26): Error: cannot implicitly convert expression rhs of type immutable(Deb) to Deb
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/variant.d(603,17): Error: template instance std.variant.VariantN!32LU.VariantN.opAssign!(immutable(Deb)) error instantiating
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/concurrency.d(126,22):        instantiated from here: __ctor!(immutable(Deb))
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/concurrency.d(656,23):        instantiated from here: __ctor!(immutable(Deb))
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/concurrency.d(647,10):        instantiated from here: _send!(immutable(Deb))
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/../import/std/concurrency.d(626,10):        instantiated from here: _send!(immutable(Deb))
src/model.d(115,21):        instantiated from here: 
send!(immutable(Deb))
/home/mark/opt/ldc2-1.20.0-linux-x86_64/bin/ldc2 failed with exit 
code 1.
: