Incorrect "assignment to val" error when assigning to a `def`

[SethTisue]

(noticed while working on #22671 at a spree)

Minimized code

Welcome to Scala 3.6.4 (17.0.14, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                    
scala> def x = 3
def x: Int
                                                                                                    
scala> x = 4

Output

-- [E052] Type Error: ----------------------------------------------------------
1 |x = 4
  |^^^^^
  |Reassignment to val x

Expectation

x isn't a val, it's a def, so it's nonsensical for the error to refer to "val x"

[SethTisue]

Scala 2 takes a different tack (we don't necessarily need to do it the same in Scala 3)

scala 2.13.16> object O { def x = 3 }
object O

scala 2.13.16> O.x = 5
                 ^
               error: value x_= is not a member of object O

[som-snytt]

I think mentioning "explicit setter" would be useful in the "explanation". I don't know whether the explanation is used a lot or never. I certainly never re-run with -explanation, what a hassle. Does the IDE use it? That would be ideal (IDEal).

Also, I suggested that this is a duplicate of the other "improve the message" ticket, but it's also fine to leave it open, especially if #22880 follows up with better improvements.