TypeScript Language Specification v1.5

3.5 Type Parameters
A type parameter represents an actual type that the parameter is bound to in a generic type reference or
a generic function call. Type parameters have constraints that establish upper bounds for their actual type
arguments.
Since a type parameter represents a multitude of different type arguments, type parameters have certain
restrictions compared to other types. In particular, a type parameter cannot be used as a base class or
interface.
3.5.1 Type Parameter Lists
Class, interface, and function declarations may optionally include lists of type parameters enclosed in <
and > brackets. Type parameters are also permitted in call signatures of object, function, and constructor
type literals.
TypeParameters:
< TypeParameterList >
TypeParameterList:
TypeParameter
TypeParameterList , TypeParameter
TypeParameter:
Identifier Constraint opt
Constraint:
extends Type
Type parameter names must be unique. A compile-time error occurs if two or more type parameters in
the same TypeParameterList have the same name.
The scope of a type parameter extends over the entire declaration with which the type parameter list is
associated, with the exception of static member declarations in classes.
Each type parameter has an associated type parameter constraint that establishes an upper bound for
type arguments. Omitting a constraint corresponds to specifying the empty object type {}. Type
parameters declared in a particular type parameter list may not be referenced in constraints in that type
parameter list.
The base constraint of a type parameter T is defined as follows:
If T has no declared constraint, T's base constraint is the empty object type {}.
If T's declared constraint is a type parameter, T's base constraint is that of the type parameter.
Otherwise, T's base constraint is T's declared constraint.
In the example
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