Generated logarithmic conversions still null-check a parameter that is now a value type, boxing on every call

[matt-edmondson]

What happens

Commit 4ff1d42 — "[major] A quantity is a value type" (2026-09-09) — turned all 212 generated quantities into readonly record struct, explicitly to remove allocations (FromMeter 48.0 -> 0.0 bytes/op).

It did not touch LogarithmicScalesGenerator.cs, which still emits a reference-type guard:

// Semantics.SourceGenerators/Generators/LogarithmicScalesGenerator.cs:149-152
cb.WriteLine($"public static {fullType} {fromName}({linear}<T> linear)");
using (new Scope(cb))
{
    cb.WriteLine("ArgumentNullException.ThrowIfNull(linear);");
    cb.WriteLine("double linearValue = double.CreateChecked(linear.Value);");

linear is Concentration<T> / Gain<T> / Ratio<T> / SoundPressure<T> and friends — all now value types. Confirmed:

// Semantics.Quantities/Generated/…/Concentration.g.cs:12
public readonly partial record struct Concentration<T> : IVector0<Concentration<T>, T>, IPhysicalQuantity<Concentration<T>, T>

ArgumentNullException.ThrowIfNull(object?, string?) therefore takes an implicit boxing conversion of a non-nullable value type and compares the box to null.

Failure scenario

The check can never fire — it is unreachable on every one of the nine emitted sites: PH.g.cs:39, Decibels.g.cs:39 and :61, plus Cents, Semitones, SoundPressureLevel, SoundIntensityLevel, SoundPowerLevel, DirectionalityIndex.

The compiler emits a box for each call, so every Decibels.FromGain(...), SoundPressureLevel.FromSoundPressure(...) and so on carries an allocation on the hot path — precisely the per-call allocation the value-type commit was written to eliminate, and the one its benchmark suite does not cover for these types.

Why it matters

Two things, neither large on its own:

• It is dead code that reads as a live contract. A consumer reading the generated source reasonably concludes null is a case worth handling here.
• It is a measurable regression against the stated goal of the most recent major change, sitting in committed generated source that ships as written.

Suggested fix

Delete LogarithmicScalesGenerator.cs:151. If a guard is wanted for a future reference-typed linear side, emit it conditionally on the linear type actually being a reference type.

Extend QuantityValueTypeTests' allocation assertions to cover at least one logarithmic conversion, so the generator and the value-type decision cannot drift apart again — that is what let this survive the original commit.

[matt-edmondson]

Triage

Priority: Low Effort: Small

What needs to be done: LogarithmicScalesGenerator still emits ArgumentNullException.ThrowIfNull(linear) for a linear parameter that commit 4ff1d42 turned into a readonly record struct — the check is dead (boxes the value type to compare against null, which can never be true) and, worse, the boxing conversion itself allocates on every call of the nine affected generated conversions, directly regressing the allocation-elimination goal that same commit was written to achieve.

Suggested next steps / acceptance criteria:

• Delete the ArgumentNullException.ThrowIfNull(linear) line in LogarithmicScalesGenerator.cs:151.
• Extend QuantityValueTypeTests' allocation assertions to cover at least one logarithmic conversion (e.g. Decibels.FromGain), so the generator and the value-type decision can't silently drift apart again — this is exactly the coverage gap that let it survive the original commit.
• Regenerate the nine affected .g.cs files and confirm the diff is exactly the expected line removals.

Blockers / dependencies: None identified. Trivial, mechanical fix.

---

Generated by Claude Code