I'm not sure if libcxx is asserting UTF-8 here; but on Windows the full char value is always passed through in its entirety, since the default codepage is something like Windows-1252. The replacement character is only used for non-chars there; and that should be a more portable test everywhere.
(Still pending review at https://reviews.llvm.org/D47395 which has been open since may; will ask for forgiveness rather than permission :) )
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339213 91177308-0d34-0410-b5e6-96231b3b80d8
