Fix #10538 - Cannot swap a std.typecons.Tuple

[ntrel]

Avoid CTFE unsafe cast by swapping elements of tupleof.

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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#8864"

[schveiguy]

this needs effort to make it pass CI. It should not be automerged by accident. Should be reviewed after being fixed.

[thewilsonator]

Please retitle the commit message "Fix bugzilla issue 24285 - ..."

[LightBender]

@ntrel typecons is coming up for Phobos 3. Is there any chance we can get this merged soon so that we don't have to duplicate this fix?

[0xEAB]

Backup copy of the previous commits on the branch: https://github.com/0xEAB/phobos/tree/ntrel/swap-ctfe-backup
See also, https://github.com/0xEAB/phobos/tree/0xEAB/swap-ctfe-backup for an intermediate state of my edits to the patch.

[0xEAB]

AFACIT, apart from the sumtype breakage that is pointed out by CI, this fails at least this test:

phobos/std/algorithm/sorting.d

Line 2145 in 336bed6

static assert(hasSwappableElements!R);

[0xEAB]

@ntrel
What was this overload supposed to solve?

[0xEAB]

Unfortunately, I haven’t been able to figure that out myself.
All I can tell is that it breaks this code:

phobos/std/algorithm/sorting.d

Line 2145 in 336bed6

static assert(hasSwappableElements!R);

[0xEAB]

This was awful to debug. I’d love if someone could track this down and open a DMD bug report.