The where clause can be used to define more complex type constraints, for instance, to conform to more than one protocol with some constraints.
We can specify additional requirements on type parameters and their associated types by including a where clause after the Generic parameter list. A where clause consists of the where keyword, followed by a comma-separated list of one or more requirements.
For instance, we can express the constraints that a Generic type T inherits from a C class and conforms to a V protocol as <T where T: C, T: V>.
We can constrain the associated types of type parameters to conform to protocols. Let's consider the following Generic parameter clause:
<Seq: SequenceType where Seq.Generator.Element: Equatable>
Here, it specifies that Seq conforms to the SequenceType protocol and the associated Seq.Generator.Element type conforms to the Equatable protocol. This constraint ensures that each element of the sequence is Equatable.
We can also specify that two types should be identical using the == operator. Let's consider the following Generic parameter clause:
<Seq1: SequenceType, Seq2: SequenceType where Seq1.Generator.Element == Seq2.Generator.Element>
Here, it expresses the constraints that Seq1 and Seq2 conform to the SequenceType protocol and the elements of both sequences must be of the same type.
Any type argument substituted for a type parameter must meet all the constraints and requirements placed on the type parameter.
We can overload a Generic function or initializer by providing different constraints, requirements, or both on the type parameters in the Generic parameter clause. When we call an overloaded Generic function or initializer, the compiler uses these constraints to resolve which overloaded function or initializer to invoke.