Concepts (lite, the version that is still around) are "one-way." They constrain what types you can use as template arguments, but they do not constrain what tem

Concepts (lite, the version that is still around) are "one-way." They constrain what types you can use as template arguments, but they do not constrain what templates can do. Which means the templates themselves can still impose additional requirements based only on their internals, leading to the same bad error messages for users and lack of checking for library authors that we have without concepts.