Using vs Typedef

What this C++ alias syntax question tests

This is a medium-difficulty question on modern C++ language semantics. It probes whether a candidate understands the practical and theoretical differences between two ways to declare type aliases, and recognizes that newer language features are not always strict supersets of older ones.

Post-C++11, using declarations became an alternative to typedef for aliasing. Candidates often assume that using is simply better or more flexible, but this question asks you to think critically about edge cases—scenarios where one mechanism can express something the other cannot. The answer requires reasoning about template parameters, scope rules, and the full range of what each syntax permits.

• Type alias declarations and their syntax
• Template type parameters vs. template template parameters
• Scope and forward-declaration rules for aliases
• Backwards compatibility and legacy code patterns