Implicit conversions of head-const to tail-const ranges

[Per Nordlöw]

On Tuesday, 23 March 2021 at 20:44:03 UTC, Paul Backus wrote:
> You can't do a naive `const(Range!T)` -> `Range!(const(T))` 
> conversion, because there may be `static if` statements (or 
> other reflection) inside the `Range` template that cause 
> incompatible structs to be generated for `T` and `const(T)`.

I don't quite follow what this refers to. Can you give a code 
reference or sample?