Leading vs Trailing Requires-Clause

Understanding leading and trailing requires-clauses in C++20

This is a medium-difficulty C++ language question that tests your grasp of requires-clauses — the mechanism C++20 introduced for expressing constraints on templates. It probes whether you understand the syntactic and semantic differences between two ways of writing the same logical constraint.

The question centres on a real stumbling block for developers learning constrained templates: the distinction between where a requires-clause can appear in the function signature, and how that placement interacts with template parameters and non-template member functions. Getting this wrong leads to code that looks correct but fails to compile, making it a practical test of language semantics rather than just theory.

• Requires-clause placement (leading vs. trailing position)
• Template parameter scope and constraint visibility
• Substitution and overload resolution in constrained contexts
• Member function template constraints