Component/s: Standard Library
Xcode 10.3 (10G8)
macOS 10.14.6 (18G87)
See this forum post:
and the attached Xcode project.
(EDIT: Never mind the attached project, the bug is easily demonstrated by the example in the forum post. I've now deleted the attachment to prevent unnecessary confusion.)
In short, if compiling with optimizations enabled, the following Character initializer will not trap (as it should) when the given string is invalid (ie the string does not hold exactly one extended grapheme cluster).
The result is that very strange Character instances can be created without any notice.