[Issue 6125] to!string doesn't throw on invalid UTF sequence

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=6125

--- Comment #4 from berni44 <bugzilla at d-ecke.de> ---
The original bug isn't windows specific. I don't know if the example from Lucia
Cojocaru can be considered the same bug...

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