added to type mismatch reporting output of tree, which can't be typed

[rssh]

added to type mismatch reporting output of tree, which can't be typed in -explain mode.
Because in macros, tree in sourcecode in position and a processed tree can be very different.

[SethTisue]

Looks plausible, but needs test coverage, yes? (In part because a test or two would help illustrate the purpose of the PR. Perhaps that's why it hasn't attracted reviewer attention so far.)

[dwijnand]

I agree, the intent seems reasonable to me, to help understand what went wrong in the expanded macro code. But we'd want to see some examples as tests to review and maintain this enhancement.

[rssh]

Just added tests. will rebase and squash now.
Short description of test-case here:

package t12717

import scala.quoted._

object A:

   def foo(x:Int): Int = ???

   def bar(x:String): String = ???


object X:

   inline def doSomething[T](inline x:T):Any = ${
      doSomethingImpl('x)
   }

   def doSomethingImpl[T:Type](x:Expr[T])(using Quotes):Expr[Any] =
     import quotes.reflect._
     val aTerm = '{A}.asTerm
     val xBar = Apply(Select.unique(aTerm,"bar"),List(x.asTerm))
     Apply(Select.unique(aTerm,"foo"), List(xBar)).asExpr

And usage:

package t12717

object Test:

   val x = X.doSomething("XXX") // error

The output is:

-- [E007] Type Mismatch Error: tests/neg-custom-args/hidden-type-errors/Test.scala:6:24 --------------------------------
6 |   val x = X.doSomething("XXX") // error
  |           ^^^^^^^^^^^^^^^^^^^^
  |           Found:    String
  |           Required: Int
  | This location contains code that was inlined from Test.scala:6

Explanation
===========

Tree: t12717.A.bar("XXX")

I tried to show that
  String
conforms to
  Int
but the comparison trace ended with `false`:


  ==> String  <:  Int
    ==> String  <:  Int (recurring)
      ==> String  <:  Int (recurring)
      <== String  <:  Int (recurring) = false
    <== String  <:  Int (recurring) = false
  <== String  <:  Int = false

The tests were made under the empty constraint

1 error found

Without

Tree: t12717.A.bar("XXX")

it's hard to understand, why the type checking of macro result has failed.

[rssh]

btw, also I'm not sure that I added the test in the right place: looks like a neg-custom-args check only the existence of error, not an exact output of the compiler. Maybe we need a new category for such test types (?)

[dwijnand]

Seems reasonable to me. What I see missing is that Int is expected because that tree is being passed to A.foo, which is still only understandable from reading the macro. So it almost seems like a half improvement, but that's still an improvement over the status quo.

I wouldn't mind another maintainer weighing in with or against me, before merging.