[ZainnQureshii]

Summary

checked_div and checked_mod in folly/lang/CheckedMath.h only reject divisor == 0. For a signed built-in type at least as wide as int (no promotion), min / -1 and min % -1 are undefined behaviour (the quotient does not fit), so checked_div(&r, INT64_MIN, int64_t{-1}) runs the division instead of returning false. UBSan reports division of -9223372036854775808 by -1 cannot be represented in type 'long long' at both call sites.

Both functions now return false with *result = {} for that pair, the same way they handle a zero divisor. The check is under if constexpr for built-in signed integers only, using folly::is_integral_v and folly::is_signed_v so __int128 is covered on standard libraries whose std:: traits leave it out (libstdc++ in strict -std=c++XX mode). The minimum is computed from the sign bit via folly::make_unsigned_t rather than std::numeric_limits, so the guard also works where strict-mode libstdc++ (GCC 10.1 and 10.2) leaves numeric_limits<__int128> unspecialised, which would make min() return 0 and reject 0 / -1. Class types keep their old behaviour. checked_mod tests the promoted operand type, so int8_t(-128) % int8_t(-1) (computed in int) still returns true with 0.

One visible change for narrow types: checked_div on int8_t(-128) / int8_t(-1) or int16_t(-32768) / int16_t(-1) used to return true with the wrapped value (-128, -32768), because the division happens in int and only the store narrows. It now returns false, since the true quotient does not fit in T.

The test checkedModZeroDivisor was calling checked_div; it now calls checked_mod. Because the header now uses folly/Traits.h, checked_math lists //folly:traits in folly/lang/BUCK and folly_traits in folly/lang/CMakeLists.txt, and the test target lists it in folly/lang/test/BUCK.

Test plan

folly/lang/test/CheckedMathTest.cpp built with clang++ -std=c++20 -fsanitize=undefined against GoogleTest:

• Original header, new tests kept: checkedDivSignedOverflow, checkedModSignedOverflow, checkedDivNarrowSignedOverflow and checkedDivModInt128SignedOverflow fail, with UBSan overflow reports at CheckedMath.h:185 and :195. Result: 41 tests pass and four fail, with six UBSan overflow reports.
• Fixed header: all 45 tests pass with no UBSan reports (built with -DFOLLY_NO_CONFIG -DFOLLY_HAVE_INT128_T=1 so the __int128 tests run), including the new checkedDivModInt128ZeroDividend, which checks that 0 / -1 and 0 % -1 still succeed.

The full CMake/CTest build, Buck, native GCC builds and other platforms were not run. The GCC 10.1, 10.2, 10.3 and 11.1 <limits> headers were compiled with Clang in small probes (strict and GNU C++17/C++20) to check the __int128 specialisation, and libc++ probes passed in the same modes.

AI assistance

Prepared with AI assistance (Claude Code) and reviewed with GitHub Copilot.

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