Type instability in ForwardDiff.gradient

Specific Domains Numerics
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9

No worries, I didn’t try to actually execute it at first either. This seems to work, and the reason I thought it didn’t above seems to be that I’d defined the GradientConfig in the global scope. In the example below g1 is not type-stable, but g2 is.

using ForwardDiff, LinearAlgebra

x = randn(100)
f(x) = dot(x, x)

cfg = ForwardDiff.GradientConfig(f, x, ForwardDiff.Chunk{2}())
g1(x) = ForwardDiff.gradient(f, x, cfg)
g2 = let cfg = cfg
    x -> ForwardDiff.gradient(f, x, cfg)
end
@code_warntype g1(x)
@code_warntype g2(x)

I’ll need to check my original, non-MWE code, but suspect the issue there may have also been scoping-related.