Methods named `extension` cannot be invoked

[tgodzik]

Minimized code

This code doesn't compile:

def extension(a : Int ) = 5 
extension(1) 

Expectation

Either this compiles or we shouldn't be able to name the function extension

[nicolasstucki]

extension is parsed as an extension clause.

We need to make followingIsExtension a bit smarter.
https://github.com/lampepfl/dotty/blob/3a52817bf06e1099a3bbf274fe6566760a56ecb4/compiler/src/dotty/tools/dotc/parsing/Parsers.scala#L918-L920

Maybe we should make sure there is a space between the extension and the arguments in extension (x: T) def ... for extension clauses. extension(x: T) def ... seem wrong. If that is the case the application would not be ambiguous.

[odersky]

What if the function is parameterless? What if it is meant to be called like this?

extension{1}

Or it might be intended to be used as an infix operator. In all of these cases extension would still be callable.

So I think it's best to leave it as it is. We want to have a simple criterion when extension is a keyword and that's that it is followed by (.

[tgodzik]

@odersky is it decided that extension will become a keyword? I will need to update it in the scalameta parser.

[nicolasstucki]

Yes, extension will be a keyword. It is the only possible way to make parsing unambiguous for this kind of syntax.

[tgodzik]

extension is still not a keyword and the issue is still not fixed. I order to properly support it in scalameta I need to be sure what the situation is. Please do let me know what is going on with this.

[odersky]

extension will stay as a soft keyword only. I have just updated the file on soft keywords in #10878:

https://github.com/lampepfl/dotty/pull/10878/files#diff-e6b4e879e6047adb52942d05da6e5900f1ffdcd705e8e044d5cd169e13532749

That one explains when extension is treated specially:

Otherwise, soft keywords are treated specially in the following situations:
... - extension, if it appears at the start of a statement and is followed by ( or [

Since that's what's actually implemented, I think we can close the issue now.