Add forward mode rules for calling a FunctionWrappersWrapper - #84
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ChrisRackauckas merged 4 commits intoAug 18, 2026
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AstitvaAggarwal
marked this pull request as draft
August 10, 2026 12:19
AstitvaAggarwal
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August 10, 2026 14:19
build_frule(f_orig_dual, args...) is an existing convenience method that extracts the interpreter/signature internally, same as build_rrule(sig) already does for the reverse-mode rule above. Removes the need to import ForwardMode/get_interpreter at all, cutting the Mooncake QA ignore list from 15 to 13 symbols.
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Mooncake had a reverse mode rule for calling a FunctionWrappersWrapper, but no forward mode rule, so forward mode differentiation through code that calls one (for example, second derivatives) failed.
This adds a forward mode rule for calling a wrapper, plus rules for
unwrap, which reaches into a nested FunctionWrapper's internal field in a way the generic getfield rule cannot handle since FunctionWrapperTangent is a custom tangent type.Every rule here builds a fresh zero tangent for the unwrapped function rather than threading through any real tangent it might carry. That is only correct if the wrapped function itself has nothing to differentiate; if it does (for example, a struct holding a learnable matrix), it now errors clearly instead of silently returning a too-small gradient. Derived rules are cached by signature too, since the wrapped function is typically called once per ODE timestep, and rebuilding a rule from scratch on every call would be wasted work.
Needed for SciML/SciMLSensitivity.jl#1427.