hessian!(::ImmutableDiffResult, f, ::StaticArray) errors when f does not depend on its argument

hessian!(::ImmutableDiffResult, f, ::StaticArray) errors outright whenever f returns no Dual of its own tag.

using ForwardDiff, StaticArrays, DiffResults
sx = SVector(1.0, 2.0, 3.0)

# (a) constant `f`
ForwardDiff.hessian!(DiffResults.HessianResult(sx), z -> 2.0, sx)
# ERROR: DimensionMismatch: No precise constructor for SMatrix{3, 3, Float64, 9} found.
#        Size of input was (0, 3).

# (b) `f` constant in `z` but carrying an enclosing tag
ForwardDiff.derivative(1.0) do a
    r = ForwardDiff.hessian!(DiffResults.HessianResult(sx), z -> a * 2.0, sx)
    DiffResults.value(r)
end
# ERROR: MethodError: no method matching extract_jacobian(::Type{Tag{…}}, ::Dual{…}, ::SVector{3, Float64})

Every other path handles both fine — hessian(f, sx), hessian!(::Matrix, f, sx) and hessian(f, ::Vector) all return a 3×3 zero Hessian, and the nested forms of those return 0.0.

Cause

The method extracts with extract_jacobian(T, partials(T, fd2), x), and extract_jacobian(::Type{T}, ydual::Union{StaticArray,Partials}, x) assumes partials(T, fd2) has one entry per input. It does not when f returns no Dual{T}:

• a constant gives Partials{0}, so the result is built at size (0, N) and the SMatrix constructor rejects it;
• a Dual{S} with S ≺ T gives zero(d), a bare Dual, which matches no method at all.

Suggested fix

Dispatch on the result rather than on its partials, returning a zero length(x) × length(x) Hessian when there is no differentiated layer to read — a constant f, an empty x, and a result that carries only an enclosing tag all fall under the same rule.

Version: ForwardDiff master (v1.4.5), Julia 1.12.7, StaticArrays 1.9.19.

Start at the hessian! implementation path that calls extract_jacobian(T, partials(T, fd2), x), then trace how constant results and enclosing-tag Dual results are handled. Add regression coverage for both examples, including the empty-input case, and verify that each produces the expected zero Hessian or nested scalar result.