Consider:
1. File struct.swift defining `struct Struct` with `static func max` member
2. File derivatives.swift defining `extension Struct` with custom derivative of the `max` function
3. File error.swift defining a differentiable function which uses `Struct.max`.
Previously, when passing error.swift as primary file and derivatives.swift as a secondary file to swift-frontend (and forgetting to pass struct.swift as a secondary file as well), an assertion failure was triggered.
This patch fixes the issue by adding a check against `ErrorType` in `findAutoDiffOriginalFunctionDecl` before calling `lookupMember`.
Co-authored-by: Anton Korobeynikov <anton@korobeynikov.info>
* Lookup for custom derivatives in non-primary source files after typecheck is finished for the primary source.
This registers all custom derivatives before autodiff transformations and makes them available to them.
Fully resolves#55170
