From https://d.puremagic.com/issues/show_bug.cgi?id=11966 :
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> what does 'inout const' mean?
- A common type qualifier between immutable and inout
- If inout function returns inout(const(T)), it will be deduced to const(T),
immutable(T), or inout(const(T)). Different from inout(T), it won't be deduced
to T because 'const' sticks.
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Should be in function.dd