Fix issue 13930, 19345 - Fix receiveOnly for non-assignable types

[tom-tan]

This request fixes the following two issues:

• Fix issue 13930 - std.concurrency can't send immutable AA to another thread
• Fix issue 19345 - std.concurrency does not work with structs of const value type

I used a different issue title for this request from the above two issues because the source of the problem is about receiveOnly rather than send function or std.concurrency itself.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	13930	normal	std.concurrency can't send immutable AA to another thread
✓	19345	normal	std.concurrency does not work with structs of const value type

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7655"

[thewilsonator]

please rewrite the commit message to say fixes issues 13930, 19345 so the bot picks them up. (note the cross next to auto-close).

[tom-tan]

@thewilsonator Thank you for your advice. I updated the commit message.

[tom-tan]

Thanks!

[Geod24]

This fails on Win32:

"D:\a\1\dmd\generated\Windows\RelWithAsserts\Win32\dmd.exe" -conf= -g -m32mscoff -O -w -preview=dip1000 -transition=complex -I../druntime\import -unittest -version=StdUnittest -version=CoreUnittest -c  -ofunittest3c.obj std\signals.d  std\meta.d  std\typetuple.d  std\traits.d  std\encoding.d  std\xml.d  std\random.d  std\exception.d  std\compiler.d  std\system.d  std\concurrency.d
std\variant.d(703): Error: cannot modify struct instance `*p` of type `Aggregate` because it contains `const` or `immutable` members
std\variant.d(604): Error: template instance `std.variant.VariantN!20u.VariantN.opAssign!(const(Aggregate))` error instantiating
std\concurrency.d(126):        instantiated from here: `__ctor!(const(Aggregate))`
std\concurrency.d(660):        instantiated from here: `__ctor!(const(Aggregate))`
std\concurrency.d(650):        instantiated from here: `_send!(const(Aggregate))`
std\concurrency.d(627):        instantiated from here: `_send!(const(Aggregate))`
std\concurrency.d(2756):        instantiated from here: `send!(const(Aggregate))`

I have no idea why it doesn't fail on other platform, but this needs to be reverted. I'm also going to set Azure as required so that the bot doesn't merge failing PRs. Sorry @tom-tan , could you re-submit a PR after the revert ?

[tom-tan]

@Geod24 Done! See #7661.