Wrong error message when using setter extension method with wrong type

[keynmol]

Compiler version

3.4.1

Minimized example

//> using scala 3.4.1

object types:
  opaque type Struct = Int
  val test: Struct = 25
  extension (s: Struct) 
    def field: Int = s
    def field_=(other: Int) = ()

@main def hello = 
  import types.*
  test.field = "hello"

Output Error/Warning message

[error] ./test.scala:12:3
[error] Reassignment to val field
[error]   test.field = "hello"
[error]   ^^^^^^^^^^^^^^^^^^^^

Why this Error/Warning was not helpful

I expected there to be any information about type mismatch in the error message.

-explain doesn't help:

[error] Explanation
[error] ===========
[error] You can not assign a new value to field as values can't be changed.
[error] Keep in mind that every statement has a value, so you may e.g. use
[error]   val field = if (condition) 2 else 5
[error] In case you need a reassignable name, you can declare it as
[error] variable
[error]   var field = ...
[error]   test.field = "hello"
[error]   ^^^^^^^^^^^^^^^^^^^^

[som-snytt]

scala 2 got extra help for erroneous rewrite to assignment op:

Welcome to Scala 2.13.14 (OpenJDK 64-Bit Server VM, Java 21.0.2).
Type in expressions for evaluation. Or try :help.

scala> object x { def apply() = 42 ; def update(i: Int) = () }
object x

scala> x() += "42"
           ^
       error: value += is not a member of Int
         Expression does not convert to assignment because:
           type mismatch;
            found   : String
            required: Int
           expansion: x.update(x.apply().+("42"))

scala>

compare

Welcome to Scala 3.4.1 (21.0.2, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> object x { def apply() = 42 ; def update(i: Int) = () }
// defined object x

scala> x() += "42"
-- [E008] Not Found Error: ---------------------------------------------------------------------------------------------
1 |x() += "42"
  |^^^^^^
  |value += is not a member of Int - did you mean Int.!=? or perhaps Int.!= or Int.!= or Int.!=?
1 error found

scala>

It might be more complex when extensions are involved, but it's a similar area for reporting.

I suspect scala 2 excludes the unhelpful hint about != on other grounds, let alone recommending it thrice. (As in, "a hint so nice, it's recommended thrice!") Oh, I miscounted, it's four times.

[mbovel]

Reproduced on 3.5.0 (at 8563571e5af).

[mbovel]

This issue was picked for the Scala Issue Spree of tomorrow, May 21st. @hamzaremmal, @AnotherMedo, @iusildra, @mbovel will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.