Do and DoAndReturn panic when a variadic argument is nil #315

Actual behavior A mocked variadic method whose expectation carries a Do or DoAndReturn function panics when the code under test passes nil as a variadic element. Both actions synthesize the zero value for a nil argument from ft.In(i), where i counts the arguments flattened as the mock passes them, but a variadic func type holds the slice at its last position and nothing beyond it. At the last position the zero value is the slice itself and reflect.Call panics with reflect: cannot use []error as type error in Call; past it, ft.In(i) panics with index out of range.

Expected behavior The function receives nil in that variadic position, the same as it does for a nil fixed argument.

To Reproduce Steps to reproduce the behavior

1. Build a Call for func(string, ...error) and attach Do(func(_ string, errs ...error) {}).
2. Run the action with the arguments "msg", nil, nil.
3. The test binary panics instead of calling the function with two nil errors.

TestCall_Do_NilVariadicArg and TestCall_DoAndReturn_NilVariadicArg in the branch below do exactly this and fail on main at d8c3a2a.

Additional Information

• gomock mode (reflect or source): both, the failure is in gomock/call.go
• gomock version or git ref: main at d8c3a2a
• golang version: go1.26.2

Triage Notes for the Maintainers

I have a fix with the two tests ready at https://github.com/lenamonj/mock/commit/17c9aeeb071acccf62bc6c4e4c6912774616db16: an argTypeAt helper that folds every position from the variadic one onwards onto the element type, used at both zero-value sites. go vet, go build, ci/test.sh, ci/check_panic_handling.sh and the module tests pass on it. CONTRIBUTING asks for an issue before a pull request, so I am opening this first and will open the pull request if you want the change.