[Issue 18436] broken opCast fails silently when used with std.conv.to

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18436

--- Comment #1 from elpenguino+D at gmail.com ---
https://github.com/dlang/phobos/pull/6172 attempts to fix this, but is blocked
due to missing functionality. a trait to determine if a set of arguments match
a template WITHOUT attempting to instantiate is required.

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