Type parameter

A type parameter is a named variable ranging over admissible types in a generic definition. Instantiation substitutes a particular type argument, allowing one declaration to retain type information across uses. An upper type bound restricts the admissible arguments; a variance annotation instead describes how instantiated types participate in subtyping.

A type substituted for a type parameter when a generic declaration is instantiated. A type argument must satisfy the declaration's upper and lower bounds. It is distinct from an ordinary run-time argument supplied to a function call.