[swift-evolution] [Discussion] File-level declarations having highest priority for shadowing

Charlie Monroe charlie at charliemonroe.net
Mon Jun 20 13:54:57 CDT 2016

I've filed this previously as a compiler bug, but was told to discuss this here (https://bugs.swift.org/browse/SR-1772).


Consider the following code:

private func _validateAccount(name: String, existingAccounts: [Account]) -> Bool {
    // Logic goes here...

class MyController {
    private func _validateAccount(name: String) -> Bool {
        let accounts = self.accounts

	// Error: Extra argument 'existingAccounts' in call
        return _validateAccount(name, existingAccounts: accounts)

_validateAccount(name:, existingAccounts:) is declared at file-level since it is used in two separate controllers (my example comes from an app with accounts and the check is done during creation + when renaming) and then a similar method (with less arguments) is declared on the controllers.

Currently, the compiler gives an error about ambiguous use of _validateAccount, since it's declared on MyController as well which is taking precedence, even though the number of arguments is different (and the compiler is only checking base names, not arguments).

As Jordan Rose mentioned in a comment to my report, solution to this is to specify the function using Module._validateAccount, but this is not applicable to Playgrounds.

This is partially an issue of the ongoing discussion of various name collisions between modules, but in this particular case, it's one single file.

I propose that file-level symbols take precedence over any reference to current type's members, unless explicit self is specified, or at least when it comes to calling a method, the type checker goes up the scope hierarchy to see if there isn't a function satisfying the arguments.

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