stillwater-sc / universal / 30536515033

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Commit Message

fix: eliminate implicit limb-mask narrowing via shared bit_clear_mask helper (#1260) (#1266)

The idiom `bt null = ~(1ull << (i % bitsInBlock))` builds a 64-bit mask and
implicitly narrows it to the limb type bt in every setbit. The truncation is
intended (only the low bitsInBlock bits matter) but implicit, so GCC/Clang
-Wconversion and MSVC C4244 fire at every template instantiation site (observed
flooding stillwater-sc/mp-blas CI).

Add a dependency-free constexpr helper in internal/bit_manipulation.hpp:

    template<typename bt>
    constexpr bt bit_clear_mask(unsigned i, unsigned bitsInBlock) noexcept {
        return static_cast<bt>(~(bt(1) << (i % bitsInBlock)));
    }

making the truncation explicit (warning-free) in ONE place, and route all sites
through it. The issue listed 5 files; the identical idiom was actually in 10
(blockbinary, blockfraction, blocksignificand, cfloat, lns, posit_exponent,
integer, takum, areal, dbns) -- all converted so the idiom cannot drift into the
next number system. posit_exponent's std::uint32_t variant uses
bit_clear_mask<std::uint32_t>(i, 32).

Verified: the helper is -Wconversion/-Werror clean on gcc + clang and produces
the identical mask for every bit position of uint8/16/32/64 (new regression
internal/blockbinary/api/bit_clear_mask.cpp, with constexpr static_asserts). All
10 consumers compile + run clean on both compilers; bb_api, cfloat_api/assignment,
posit_api/conversion, lns_api, areal_api PASS (setbit behavior unchanged).

Resolves #1260

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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