Type inference not working for extension method using prefixless extension

[prolativ]

Compiler version

3.4.0-RC1-bin-20230822-3783220-NIGHTLY and before

Minimized code

extension (s: String)
  def doubled = s ++ s
  def quadrupled = doubled ++ doubled

Output

[error] FakeRecursiveExtension.scala:3:20
[error] Overloaded or recursive method quadrupled needs return type
[error]   def quadrupled = doubled ++ doubled
[error]                    ^

Expectation

I would expect the compiler to be able to infer the type of quadrupled so that the snippet above compiles successfully. Interestingly, the problem doesn't occur if the extension method is used with a prefix: def quadrupled = s.doubled ++ s.doubled.
If for some surprising reason the inference problem cannot be fixed easily, at least the error message should be improved.

[nicolasstucki]

Minimized

extension (s: String)
  def s1: String = "a"
  def s2 = s1
//         ^ Overloaded or recursive method s2 needs return type

[som-snytt]

Me too, but I forgot that I already clicked the heart reaction.

It doesn't matter if the extension is not "collective"; that is not surprising in that collective syntax is just a "shorthand"; it is surprising in that simple idents are prefixed if "defined in the same collective extension" (says the doc). Maybe testing that conditional incurs the cycle.

It occurs to me that "collective extension does not introduce a scope" might imply that "same collective" is indistinguishable from "extension of same type in same owner". Edit: no, a collective is identified by position of the "receiver" parameter.

Welcome to Scala 3.7.2-RC1 (17.0.15, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> extension (s: String) { def f = "hello, world"; def g = f }
-- [E044] Cyclic Error: --------------------------------------------------------
1 |extension (s: String) { def f = "hello, world"; def g = f }
  |                                                        ^
  |                        Overloaded or recursive method g needs return type
  |
  |                         Run with -explain-cyclic for more details.
  |
  | longer explanation available when compiling with `-explain`
1 error found

scala> object X { extension (s: String) def f = "hello, world"; extension (s: String) def g = f(s) }
-- [E044] Cyclic Error: --------------------------------------------------------
1 |object X { extension (s: String) def f = "hello, world"; extension (s: String) def g = f(s) }
  |                                                                                       ^
  |                        Overloaded or recursive method g needs return type
  |
  |                         Run with -explain-cyclic for more details.
  |
  | longer explanation available when compiling with `-explain`
1 error found

scala> object X { extension (s: String) def f = "hello, world"; extension (s: String) def g = this.f(s) }
// defined object X

scala> object X { extension (s: String) def f = "hello, world"; extension (s: String) def g = s.f }
// defined object X