Before C++20 templates were untyped. Since C++20 it's possible to define structural types for templates via concepts. Concepts are an explicitly structural type

Before C++20 templates were untyped. Since C++20 it's possible to define structural types for templates via concepts. Concepts are an explicitly structural type system for templates. Nominal typing of generic code would be what most other languages that supports generic programming use. For example Rust has traits where one defines a named trait and must explicitly specify that a type satisfies a trait by name. In C++ there is no such means, a concept does strictly structural checks (via constraints) to determine if a template satisfies a concept, no names are involved. In other words, a type T satisfies a concept C defined by a constraint R if and only if a T can be substituted for C's placeholder type during the evaluation of R.