swap-and-destroy called too eagerly

[WalterBright]

The error exhibits as:

std/typecons.d(984): Error: none of the overloads of template `std.algorithm.mutation.swap` are callable using argument types `!(Tuple!(ubyte, uint))(Tuple!(ubyte, uint), Tuple!(immutable(ubyte), uint))`
std/algorithm/mutation.d(2840):        Candidates are: `swap(T)(ref T lhs, ref T rhs)`
std/algorithm/mutation.d(3102):                        `swap(T)(ref T lhs, ref T rhs)`
  with `T = Tuple!(ubyte, uint)`
  must satisfy the following constraint:
       `is(typeof(lhs.proxySwap(rhs)))`
iteration.d(69): Error: template instance `std.typecons.Tuple!(ubyte, uint).Tuple.opAssign!(Tuple!(immutable(ubyte), uint))` error instantiating
iteration.d(13):        instantiated from here: `Group!("a == b", immutable(ubyte)[])`
iteration.d(109):        instantiated from here: `group!("a == b", immutable(ubyte)[])`

triggered by dlang/dmd#16314 and is blocking it.

The trouble is Tuple.opAssign() selects swap-and-destroy when one of the arguments is const/mutable/shared, and so cannot be assigned to. This fix causes the opAssign branch to be taken instead.

The test case is a bit difficult to come up with, as it comes with 16384.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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