On Sunday, 1 August 2021 at 18:21:32 UTC, Michael Galuza wrote: > > In other words, can we implement method `void S.opAssign(T > value) const` of some struct `S` which change internal state of > `S` and this method doesn't have UB and doesn't break D's type > system. No. Mutating an object typed as `const` is always UB in D.