static if (T.sizeof == 4) { /*A*/ }
else static if (T.sizeof == 8) { /*B*/ }
else static if (T.sizeof == 12) { /*C*/ }

version (Windows) {}
else version (linux) {}
else version (OSX) {}

static if (something) {}
else version (foo) {}
else static if (somethingelse) {}

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When such a construct is detected and none of the conditional blocks is matched the compiler should warn/error about an unhandled case.
We could also try to make the declaration of such a ladder without an else-clause a warning/error. This would require an explicit "else {}" to achieve the current behavior and avoids inadvertently missing else clauses similar to how non-final switch statements require a default label.