Add note about type mismatch in automatically inserted apply argument

[mbovel]

Started during the last spree with @jan-pieter and @iusildra.

Fixes #19680.

[mbovel]

This not ready yet, there would be subtleties to take care off like correctly line-wrapping the error message, but I am already adding @odersky as a reviewer for a high-level review: is the approach reasonable?

[mbovel]

The note is not always relevant, for example consider:

def Test = List(1,2)("hello")

Does that clarify anything to say that:

  | The required type comes from a parameter of the automatically
  | inserted `apply` method of `List[Int]`,
  | which is the type of `List.apply[Int]([1,2 : Int]*)`.

?

As a middle ground, I moved the message to -explain.

[mbovel]

The pickling failure in "Applications.scala" is just due to an extra line break…

➜  ~/dotty git:(mb/19680) diff before-pickling.txt after-pickling.txt
17956,17957c17956,17957
<                                                                                             dotty.tools.dotc.reporting.Diagnostic
<                                                                                               .Error
---
>                                                                                             dotty.tools.dotc.reporting.
>                                                                                               Diagnostic.Error

[odersky]

Nice improvement. Otherwise LGTM

[mbovel]

The pickling failure in "Applications.scala" is just due to an extra line break…

➜  ~/dotty git:(mb/19680) diff before-pickling.txt after-pickling.txt
17956,17957c17956,17957
<                                                                                             dotty.tools.dotc.reporting.Diagnostic
<                                                                                               .Error
---
>                                                                                             dotty.tools.dotc.reporting.
>                                                                                               Diagnostic.Error

I was able to reproduce (with scala3-compiler-bootstrapped / testOnly *BootstrappedOnlyCompilationTests -- *picklingWithCompiler), but not to minimize and find exactly where and why the line breaking mechanism differs before and after pickling.

What I tried

I tried this, but it compiles successfully with -Xprint-types -Ytest-pickler -Yprint-pos -Yprint-pos-syms:

package dotty.tools.dotc.reporting.Diagnostic
package Diagnostic

class Error

def adipiscing() =
  def elit() =
    def nunc() =
      def vel() =
        def sapien() =
          def congue() =
            def lacus() =
              def duis() =
                def quam() =
                  def eleifend() =
                    def ultricies() =
                      def morbi() =
                        def pellentesque() =
                          val dia: Any = ???
                          dia match
                            case _: String =>
                              dia match
                              case _: String =>
                                dia match
                                  case _: String => new Diagnostic.Error()
                                  case _ => dia
                              case _ => dia
                            case _ => dia
                        pellentesque()
                      morbi()
                    ultricies()
                  eleifend()
                quam()
              duis()
            lacus()
          congue()
        sapien()
      vel()
    nunc()
  elit()

@main def Test = adipiscing()

As a workaround, I just moved maybeAddInsertedApplyNote to the outer scope, which avoids the problem.

[mbovel]

@odersky: I removed the last line of the message, as discussed.