given instance is declared as erased, but is in fact used

[ghostdogpr]

Compiler version

3.1.0

Problem

This is a follow-up to #13044.

There are few occurrences where I observe the error given instance is declared as erased, but is in fact used, but increasing -Xmax-inlines doesn't solve the problem. Instead, the compilation goes on forever while using a lot of CPU.

To reproduce, clone this branch: https://github.com/ghostdogpr/caliban/tree/repro-1 (branch = repro-1)
In sbt: ++3.1.0 (the project cross-compiles and uses Scala 2 by default), then federation / compile.
You should get this:

[error] -- Error: /Users/pierre/GIT/caliban/federation/src/main/scala/caliban/federation/Federation.scala:103:66 
[error] 103 |    implicit val querySchema: Schema[R, Query] = genericSchema.gen[Query]
[error]     |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |         given instance gen is declared as erased, but is in fact used
[error]     | This location contains code that was inlined from SchemaDerivation.scala:29
[error]     | This location contains code that was inlined from SchemaDerivation.scala:115
[error]     | This location contains code that was inlined from SchemaDerivation.scala:228
[error]     | This location contains code that was inlined from SchemaDerivation.scala:29
[error]     | This location contains code that was inlined from SchemaDerivation.scala:115
[error]     | This location contains code that was inlined from SchemaDerivation.scala:228

If you add -Xmax-inlines: 50 to scala options, instead of the error it compiles forever with high CPU usage.

The code for the derivation is here: https://github.com/ghostdogpr/caliban/blob/repro-1/core/src/main/scala-3/caliban/schema/SchemaDerivation.scala
Uncommenting this line and commenting import genericSchema.gen instead removes the error.
Let me know if you need more details about it. Thanks!

[nicolasstucki]

Seems like the library generates too much code while inlining. Up to -Xmax-inlines:43 it takes around 15 seconds to fail, then with -Xmax-inlines:44 it takes 70 seconds to fail and then it starts failing due to lack of memory. The code should probably be minimized to be able to figure anything out of it.

[ghostdogpr]

Is there a way to print the generated code?
I use the following code but it doesn't get printed when this error occurs:

object PrintMacPass {
  inline def apply[T](inline any: T): T = ${ printMacImpl('any) }
  def printMacImpl[T: Type](any: Expr[T])(using qctx: Quotes): Expr[T] = {
    import qctx.reflect._
    println(Printer.TreeShortCode.show(any.asTerm))
    any
  }
}

If I were able to see what it generates I could probably understand why it's generating too much code.

[nicolasstucki]

That will unfortunately not work as once the error is emitted we stop inlining. Hence that macro will not get expanded.

I instrumented a version of the compiler that will print what is being inlined. You may clone https://github.com/dotty-staging/dotty/tree/instument-inlining and then publish it locally by doing sbt scala3-bootstrapped/publishLocal. Then use 3.1.2-RC1-bin-SNAPSHOTas Scala 3 version in your build. You will need to increase-pagewidthand maybe set-color:never`. I ran it an got this output.

Or even better, transform your inline functions that use summonInline into proper macros. Then you can programmatically debug your own code.

[ghostdogpr]

Thanks, that was super useful. I can understand what is going on.

The fact that I call explicitly import genericSchema.gen gives to this given a higher priority than the instances defined in the companion object of my typeclass. So instead of reusing instances, it generate instances for everything, including stuff like List, ::, etc (see below). That explains why it works if I remove import genericSchema.gen, but then I need to explicitly call gen for the missing instances because it won't generate them otherwise. I need to figure out a way so that gen has a lower priority than the rest.

      ==> inlining genericSchema.gen[caliban.federation.Federation.RepresentationsArgs]?
        ==> inlining SchemaDerivation_this.derived[caliban.federation.Federation.RepresentationsArgs]?
          ==> inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.federation.Federation.RepresentationsArgs]?
          <== inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.federation.Federation.RepresentationsArgs] = Nil:List[Any]
          ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
            ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[List[caliban.federation.Federation._Any]]?
            <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[List[caliban.federation.Federation._Any]] = List.apply[Any]([new SerialVersionUID(value = 3L) : Any]*):List[Any]
            ==> inlining genericSchema.gen[List[caliban.federation.Federation._Any]]?
              ==> inlining SchemaDerivation_this.derived[List[caliban.federation.Federation._Any]]?
                ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                  ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[::[caliban.federation.Federation._Any]]?
                  <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[::[caliban.federation.Federation._Any]] = Nil:List[Any]
                  ==> inlining genericSchema.gen[::[caliban.federation.Federation._Any]]?
                    ==> inlining SchemaDerivation_this.derived[::[caliban.federation.Federation._Any]]?
                      ==> inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[::[caliban.federation.Federation._Any]]?
                      <== inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[::[caliban.federation.Federation._Any]] = Nil:List[Any]
                      ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                        ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.federation.Federation._Any]?
                        <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.federation.Federation._Any] = Nil:List[Any]
                        ==> inlining genericSchema.gen[caliban.federation.Federation._Any]?
                          ==> inlining SchemaDerivation_this.derived[caliban.federation.Federation._Any]?
                            ==> inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.federation.Federation._Any]?
                            <== inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.federation.Federation._Any] = Nil:List[Any]
                            ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                              ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[String]?
                              <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[String] = Nil:List[Any]
                              ==> inlining SchemaDerivation_this.recurse[("fields" : String) *: EmptyTuple.type, caliban.InputValue *: EmptyTuple.type](index$proxy10.+(1))?
                                ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue]?
                                <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue] = Nil:List[Any]
                                ==> inlining genericSchema.gen[caliban.InputValue]?
                                  ==> inlining SchemaDerivation_this.derived[caliban.InputValue]?
                                    ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                                      ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue.ListValue]?
                                      <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue.ListValue] = Nil:List[Any]
                                      ==> inlining genericSchema.gen[caliban.InputValue.ListValue]?
                                        ==> inlining SchemaDerivation_this.derived[caliban.InputValue.ListValue]?
                                          ==> inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.InputValue.ListValue]?
                                          <== inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[caliban.InputValue.ListValue] = Nil:List[Any]
                                          ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                                            ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[List[caliban.InputValue]]?
                                            <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[List[caliban.InputValue]] = List.apply[Any]([new SerialVersionUID(value = 3L) : Any]*):List[Any]
                                            ==> inlining genericSchema.gen[List[caliban.InputValue]]?
                                              ==> inlining SchemaDerivation_this.derived[List[caliban.InputValue]]?
                                                ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                                                  ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[::[caliban.InputValue]]?
                                                  <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[::[caliban.InputValue]] = Nil:List[Any]
                                                  ==> inlining genericSchema.gen[::[caliban.InputValue]]?
                                                    ==> inlining SchemaDerivation_this.derived[::[caliban.InputValue]]?
                                                      ==> inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[::[caliban.InputValue]]?
                                                      <== inlining SchemaDerivation_this.inline$Macros$i4(caliban.schema.macros).annotations[::[caliban.InputValue]] = Nil:List[Any]
                                                      ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                                                        ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue]?
                                                        <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue] = Nil:List[Any]
                                                        ==> inlining caliban.schema.Schema.gen[caliban.InputValue]?
                                                          ==> inlining SchemaDerivation_this.derived[caliban.InputValue]?
                                                            ==> inlining SchemaDerivation_this.recurse[m.MirroredElemLabels, m.MirroredElemTypes](SchemaDerivation_this.recurse$default$1[m.MirroredElemLabels, m.MirroredElemTypes])?
                                                              ==> inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue.ListValue]?
                                                              <== inlining SchemaDerivation_this.inline$Macros$i1(caliban.schema.macros).annotations[caliban.InputValue.ListValue] = Nil:List[Any]
                                                              ==> inlining caliban.schema.Schema.gen[caliban.InputValue.ListValue]?