The Nested Requirement Trap

Understanding nested template specialisation in C++

This medium-difficulty question tests your grasp of how C++ template specialisation works when types are nested inside class templates. It probes whether you can trace the difference between primary templates, partial specialisations, and how the compiler resolves which version of a nested type to instantiate.

The question requires you to reason carefully about template instantiation order and scope. When a nested type is defined inside a template class, its definition may depend on template parameters in ways that are easy to overlook. Understanding which specialisation the compiler chooses — and why — is crucial for writing correct generic code and debugging subtle type-system issues in C++ libraries.

• Primary template vs. partial specialisation resolution
• Dependent types and two-phase lookup
• Template instantiation and member access
• Type traits and SFINAE patterns