Making castSwitch pure

[StarCanopy]

Using 
[castSwitch](https://dlang.org/phobos/std_algorithm_comparison.html#.castSwitch) for the first time, I was dismayed that it's an impure function template, even if every handler is pure. It seems that the reason for this is because the pertinent overload of TypeInfo.opEquals uses TypeInfo.toString, which is impure (for reasons of which I'm ignorant, since it overrides toString with const, @safe, and nothrow).
It's unfortunate one seemingly can't just replace the call to 
opEquals with `typeid(a) is typeid(b)`, but opEqual's logic is 
simple, and could just be replicated with toString invocations 
replaced with TypeInfo.name.
Nonetheless, this increases fragility within castSwitch, and 
possibly sets a bad precedent, so I doubt it would be accepted.
Any thoughts, however, for fixing this and similar problems? The 
ProtoObject proposal seems dead, if that's relevant.