C++ Concept

Parser

See code. One interesting thing is that is uses Pratt parser to parse constraint expression.

Beginners to C++ concepts may be confused by the new keyword requires. For example,

template<typename T>
  requires requires (T x) { x + x; }
    T add(T a, T b) { return a + b; }

This keyword can show up in multiple places. In template definition, it is called requires-cluase. In concept definition, it is called requires-expression. Note, it is an expression in this case, so its value can be assigned. The requires-expression parser is a little bit long. See code. But its goal is clear. It parses the requirements into 4 categories: RK_Type, RK_Simple, RK_Compound, RK_Nested.