fix: popcount compiling windows clang-cl#114
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PopcountLogic<LogarithmOfGroupSize, T>::CombiningMask computed
`T(1 << HalvedGroupSize) - 1`, which performs the shift at `int`
precision because `1` is an `int`. For `LogarithmOfGroupSize = 6`,
`HalvedGroupSize` is 32 — so the expression is `1 << 32`, which is UB
per [expr.shift] (shift count not less than width of the promoted type).
GCC happens to accept this in a constant-expression context; Clang and
MSVC /std:c++20 /permissive- correctly reject it ("non-type template
argument is not a constant expression", "shift count 32 >= width of
type 'int' (32 bits)"). The result is that any translation unit that
includes <zoo/swar/SWAR.h> under MSVC with conformance mode fails to
compile, because SWAR.h's unconditional static_asserts on line 589-596
force instantiation of PopcountLogic<6, uint64_t> via logarithmFloor.
Fix: do the shift at `T`'s precision — `T(T(1) << HalvedGroupSize) - 1`.
Add a regression test that directly instantiates PopcountLogic<6, u64>.
popcount<LogNBits>() would have caught this, but on non-MSVC platforms
it routes through PopcountIntrinsic (which uses __builtin_popcountll)
and never touches PopcountLogic<6>, which is why the existing popcount
tests at <1>, <2>, <4>, <5> all passed.
jamierpond
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Fixes Windows clang-cl compilation.