Path traits and relevant path members were reworked to better support
wider range of types that are compatible with path constructors, assignment
and appending members. Added support for C++17 std::string_view,
boost::string_view and boost::container::string as the possible string
types accepted by path members.
Also extended support for types convertible to one of the string types.
Previously, user's type had to be convertible to a string with a character
type that matches the native path character type. Now all supported character
types are acceptable.
Additionally, restricted members accepting a pair of iterators to only accept
iterators whose value types are one of the supported path character types.
Lastly, path::compare and comparison operators now only accept path arguments,
relying on path conversion constructors to do the job of supporting various
source types. Also removed noexcept from compare as it is using lex_compare
and iterators internally and those can throw.
Closes https://github.com/boostorg/filesystem/issues/208.
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