This is why I wrote conf.h in the form of macros that expanded to
named structure field assignments, instead of just filling it with
named structure field assignments directly. This way, I can #include
the same file again with different macro definitions, and build up a
list of what fields were set in what config options.
This new code checks that if a config option has a default, then the
type of the default matches the declared type of the option value
itself. That's what caught the two goofs in the previous commit.
This is also the part of test_conf that I _won't_ want to delete once
I've finished with the refactoring: it can stay there forever, doing
type checking at test time that the compiler isn't doing for me at
build time.
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