[Issue 18835] [REG 2.064] to!string doesnt work on enum member starting with two underscores

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18835

--- Comment #1 from Basile B. <b2.temp at gmx.com> ---
It's because __traits(allMembers) seems to ignore members starting with double
underscores. It's used in the std.format.formatValueImpl overload specialized
for enums.

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