x3::unicode::lit does not take single unicode character literal

[saki7]

These two both work:

// defined in boost/spirit/home/x3/string/literal_string.hpp
auto const dollar_def = x3::standard::lit("$"); // double quotes

// `lit` delegates to: boost/spirit/home/x3/char/char.hpp
auto const dollar_def = x3::standard::lit('$'); // single quotes

For unicode characters, the single quoted one won't work:

// defined in boost/spirit/home/x3/string/literal_string.hpp
auto const dollar_def = x3::unicode::lit(U"$"); // double quotes

// fails to compile
auto const dollar_def = x3::unicode::lit(U'$'); // single quotes

It seems like the single char lit interfaces for unicode chars are missing in char.hpp.

I'm currently using U"double quotes" for single letter grammars, which is obviously against the optimization.

[saki7]

Dear maintainers: I'd like to PR if extending char.hpp is the correct approach. Please give me the green light.

[Kojoley]

Just open a PR if you have a fix. Do not forget about tests.