swift-mirror/userdocs/diagnostics/dynamic-callable-requirements.md

@dynamicCallable Implementation Requirements

If a type is marked with the @dynamicCallable attribute, it must provide a valid implementation of dynamicallyCall(withArguments:), dynamicallyCall(withKeywordArguments:), or both. If it fails to do so, an error will be reported at compile-time. Note that an implementation of dynamicallyCall(withKeywordArguments:) is required to support calls with keyword arguments.

To be considered valid, an implementation of dynamicallyCall(withArguments:) must:

• Be an instance method. static or class implementations are not allowed.
• Have an argument type which conforms to the ExpressibleByArrayLiteral protocol. Often, this will be the built in Array type.
• The return type of dynamicallyCall(withArguments:) may be any valid type.

To be considered valid, an implementation of dynamicallyCall(withKeywordArguments:) must:

• Be an instance method. static or class implementations are not allowed.
• Have an argument type which conforms to the ExpressibleByDictionaryLiteral protocol. This can be Dictionary, KeyValuePairs (which may be used to support duplicated keyword arguments), or some other conforming type.
• The Key associated type of the argument type must conform to the ExpressibleByStringLiteral protocol. This type is used to represent the dynamic argument keywords.
• The Value associated type of the argument type and the return type of dynamicallyCall(withKeywordArguments:) may be any valid types.