Freestanding macros cannot unbox an existential argument

[eliyap]

Motivation

SE-0382 states

Macros ... can be overloaded in the same way as functions, so long as the argument labels, parameter types, or result type differ.

I am overloading a macro to swap macro implementations based on a protocol conformance.
i.e. if MyType conforms to MyProtocol, expand to this, otherwise expand to that.

For example, we can modify #stringify to require a CustomStringConvertible argument

@freestanding(expression)
public macro stringify<SomeStringConvertible: CustomStringConvertible>(_ value: SomeStringConvertible) -> String
    = #externalMacro(module: "ExistentialOverloadMacros", type: "StringifyMacro")

public struct StringifyMacro: ExpressionMacro {
    public static func expansion(of node: some FreestandingMacroExpansionSyntax, in context: some MacroExpansionContext) -> ExprSyntax {
        guard let argument = node.argumentList.first?.expression else {
            fatalError("compiler bug: the macro does not have any arguments")
        }

        return #""\(\#(argument))""#
    }
}

Description

With a normal Swift function, a function accepting some CustomStringConvertible can implicitly unbox an argument of type any CustomStringConvertible.
This ability is described in SE-0352, Implicit Open Existentials
However, our modified #stringify cannot.

let a = 17
let b = 25

func printOut(_ value: some CustomStringConvertible) {
    print("The value was '\(value)'")
}

let convertible: any CustomStringConvertible = a + b

printOut(convertible) // ok
print("The value was \"\(#stringify(a + b))\"") // ok
print("The value was \"\(#stringify(convertible))\"")
// Error: Type 'any CustomStringConvertible' cannot conform to 'CustomStringConvertible'

Proposed solution

Similar to Swift functions, I would like macros with an argument of type some Protocol to accept a value of type any Protocol.

Alternatives considered

No response

Additional information

No response