Thanks to changes to type compatibility rules via WG14 N3007, these
functions can now be called with a compatible type even within the same
TU, which makes the -Wvisibility diagnostic too chatty to have on by
default.
So in C23 mode, -Wvisibility will only diagnose an incomplete tag type
declared in a function prototype. If the tag is defined in the
prototype, the diagnostic is silenced.
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