LGTM, thanks! Sorry for the slow reply :(

Is it possible to add a test, @mth4saurabh?

apart from lacking a test, I don't really see how this is backwards-compatible, @amueller.

This should be backwards-compatible. The degree and coef0 are only used for built-in kernels, where the default values are exactly the values that were passed here. The only situation that changes is that the behavior for gamma changes if the default value of gamma in the kernel function is not None. The only built-in kernel for which this is the case is chi2_kernel, for which None is an invalid value, so using the current default is not valid.

What should the test be? That the default behavior is gamma=1 for kernel=chi2_kernel and gamma=None for kernel=rbf?

maybe it would even be better to deprecate the three parameters here, as the behavior of them not being passed to callable is a bit weird and can lead to subtle bugs?

How about if a callable was passed and any of the parameters was not None, we error? That might error on currently running code, but probably code that had a bug?

See #9166.

#9166 was merged. Closing.