The example in conv's documentation does not compile.
Arguably, it should.
As discussed in this pull request:
https://github.com/D-Programming-Language/phobos/pull/811
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int[string][double[int[]]] a;
auto b = to!(short[wstring][string[double[]]])(a);
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src\phobos\std\conv.d(1383): Error: result[to(k1)] isn't mutable
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