[Issue 16662] Can't call std.variant.visit from a pure function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=16662

--- Comment #3 from Paul Backus <snarwin+bugzilla at gmail.com> ---
I've made some progress on this, but am unable to make VariantN.peek pure,
because this if statement

    if (type != typeid(T))
        return null;

...calls object.opEquals, which is not pure. So issue #13933 needs to be fixed
before we can solve this.

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