[Issue 23845] std.math.traits.copysign returns the wrong value

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23845

--- Comment #1 from grylliade at hotmail.com <grylliade at hotmail.com> ---
Did this slip through because unit tests are run at compile time? Because the
unit tests sure as hell should’ve caught this.

I’d also recommend keeping template parameter order consistent between the
versions of copysign(); it’s not a bug, but it could damned easily cause one if
someone’s not paying attention.

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