Added Features

• #120 string_view now has empty() method
• #122 estd::numeric namespace added with e, pi, phi
• #125 estd::visit now available for use with variant

Quality Updates & Bug Fixes

• #124 streambuf fixes including better const friendliness and seekpos/seekoff scrubbing
• #131 spanbuf improvements:
  • Now forwards initialization to underlying span. seekoff passes tests
  • ispanstream and ospanstream now sit at estd namespace level
• #132 isspace now registers linefeeds, etc.

Known Issues and Limitations

• estd::numeric namespace lacks many constants
• estd::visit multi-variant visitation order and return type is UB