toBeOdd fails on negative odd numbers (false pass with .not) #191

Priority: P1

Context

JS % keeps the sign of the left operand, so -13 % 2 === -1. The check % 2 === 1 in packages/core/src/lib/NumberAssertion.ts:237 therefore treats every negative odd number as not odd:

• expect(-13).toBeOdd() throws, even though it's the method's own JSDoc example (:220).
• expect(-13).not.toBeOdd() passes. That's a false pass: users' tests go green on wrong code.

The tests only use positive values, which is why 98% coverage never caught this. Reproduced against the built dist.

Do

• Use assertWhen: Math.abs(this.actual % 2) === 1.
• Check how toBeEven handles -0, NaN, Infinity and non-integers (1.5). Decide the behaviour and document it.
• Add tests for negative numbers and non-integers to packages/core/test/lib/NumberAssertion.test.ts (the .toBeOdd block is at line 162).
• Commit as fix(core): toBeOdd fails on negative odd numbers.

Done when

The new tests fail on main and pass on the branch.

Source: code audit of stackbuilders/assertive-ts at e81a614 (2026-10-13) plus open Vanta/Dependabot findings.