[MaxFreedomPollard]

Summary

np.copysign(x, y) returns zero whenever y is zero. It is implemented in src/library/numpy.ts as absolute(x).mul(sign(y)), and sign(0) is 0, so the magnitude of x is multiplied away.

np.copysign(np.array([5, -5]), np.array([0, 0]))
// before: [0, 0]
// numpy:  [5, 5]

sign() also cannot see the sign bit, so -0 in y came back positive.

signbit() sits a few lines above copysign() in the same file and bitcasts to read the sign bit for every value class, including -0 and NaN. Reading the sign from it instead of from sign() fixes all of these cases and needs no new machinery.

Changes

• src/library/numpy.ts: copysign() is now where(signbit(y), -abs(x), abs(x)).
• test/numpy.test.ts: the existing "copysign with zero" test asserted the old [0, 0] result, so it now asserts numpy's [5, 5]. Added a test for signed zero in y and one for infinite and NaN y.

Verification

Node 22 on macOS. I ran vitest with --no-browser, which exercises the cpu and wasm devices.

• pnpm test run test/numpy.test.ts -t "copysign" --no-browser gives 4 failed and 4 passed before the source change (the two affected tests on both devices) and 8 passed after.
• pnpm test run test/numpy.test.ts test/refcount.test.ts test/dtype.test.ts test/random.test.ts src/library/numpy/einsum.test.ts --no-browser gives 1327 passed with the change and 1323 on unmodified main, with the same 4 failures on both. Those 4 are ReferenceError: Float16Array is not defined, which is Node 22 on this machine rather than anything in the diff; CI is on Node 26.
• pnpm check, pnpm lint --max-warnings 0 and pnpm exec prettier --check are all clean.
• I compared all 64 combinations of x in [1, -1, 5, -5, 0, -0, 3.5, -3.5] with y in [0, -0, 2, -2, inf, -inf, 1e-45, -1e-45] against numpy 2.4.6. Every pair matches on both cpu and wasm, and signbit(np.copysign(np.array([0]), np.array([-1]))) is true, so the signed zero result keeps its sign.

[ekzhang]

Thank you! Sorry for overlooking that

The browser tests passed in CI across chrome, safari, firefox on windows, macos, linux