Requires requires

Understanding C++20 concepts and template constraints

This hard C++ question tests your grasp of concepts and requires clauses — the constraint mechanisms introduced in C++20 that govern which types and values can satisfy a template. It's the kind of question that separates candidates who have read the standard from those who have shipped constrained templates in practice.

Working through this requires you to parse template declarations carefully, trace how concepts compose, and understand the semantics of constraint satisfaction. You'll need to reason about whether a given type or expression meets the compile-time requirements imposed by a requires clause, and recognize common pitfalls in how constraints interact with template instantiation and overload resolution.

• Concept definition and satisfaction
• Requires clauses and their scope
• Constraint checking during overload resolution
• Subsumption and concept composition