I now see that we have an existing defcustom check that runs very late in the compilation.
Although I prefer this kind of check to be carried out during macro-expansion, doing so has the disadvantage that actual values aren't always available. On the other hand, defcustom arguments are usually constants.

Anyway, I went overboard and wrote a sizeable expansion to the current set of warnings and now also checks :type args in define-widget (see attached patch). Try it out and tell me what you think. Maybe the regexp check is too ad-hocky.

Another warning that I rather like but may give too many false positives is that of `const` and `other` types without an actual value which is then assumed to be nil. This seems to be an undocumented 'feature' but it doesn't help readability; it's often unclear whether `nil` was intended or just a result of a forgotten value.