V0 `From{Unit}` factories throw ArithmeticException for NaN instead of the documented ArgumentException

[matt-edmondson]

What's wrong

Vector0Guards.EnsureNonNegative and EnsurePositive (Semantics.Quantities/Vector0Guards.cs around lines 34 and 60) test the value with T.Sign(value). For double, float and Half, Sign(NaN) throws ArithmeticException ("Function does not accept floating point Not-a-Number values."), so the documented ArgumentException is never reached.

Repro

Speed<double>.FromMeterPerSecond(double.NaN), Mass<double>.FromKilogram(double.NaN) and Wavelength<double>.FromMeter(double.NaN) all throw ArithmeticException.

Why it matters

Four places say this should be an ArgumentException:

• the guard XML docs;
• every generated factory's XML (for example Speed.g.cs around line 101), which documents only ArgumentException;
• the Quantities docs;
• CLAUDE.md ("Throw ArgumentException for validation failures").

A caller that validates user input by catching ArgumentException gets an unhandled ArithmeticException instead. #285's suggested fix routes scalar * and / through these guards, which would spread the same exception to arithmetic results such as 0 * ∞.

Suggested fix

In both guards, check T.IsNaN(value) first and throw ArgumentException($"Magnitude must be a number; received NaN.", paramName). Then compare with value < T.Zero / value <= T.Zero rather than T.Sign.

Acceptance: add tests for both guards with double.NaN, float.NaN and Half.NaN that assert ArgumentException, plus a generated-factory test such as Speed<double>.FromMeterPerSecond(double.NaN).

[matt-edmondson]

Triage

• Category: Bug (the exception type breaks the documented contract)
• Priority: Low. NaN input is rejected either way, just with ArithmeticException instead of the documented ArgumentException. Callers that catch ArgumentException to validate input are affected. The priority goes up if Vector0 magnitudes go negative through unary minus and scalar * and /: -Temperature.FromKelvin(300) is −300 K #285's fix routes scalar arithmetic through these guards, so fix this one first or in the same PR.
• Area / suggested assignment: Semantics.Quantities (Vector0Guards.EnsureNonNegative / EnsurePositive)
• Duplicates: none found. Related: Vector0 magnitudes go negative through unary minus and scalar * and /: -Temperature.FromKelvin(300) is −300 K #285.
• In progress: no matching open PR. The only open Semantics PRs are dependency bumps (Bump MSTest.Sdk from 4.4.0 to 4.4.1 #255, Bump the ktsu group with 15 updates #323).

Suggested next step: check T.IsNaN first, then compare with < / <= rather than T.Sign. Add NaN tests for double, float and Half.

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