I was just experimenting with the behavior of givens and inline in Scala 3.2.2, and ran into the following example:
trait Max[X]:
inline def max(a: X, b: X): X
inline given maxForDoubles: Max[Double] with
inline def max(a: Double, b: Double) = if a < b then b else a
inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
inline def max(x: (A, B), y: (A, B)) =
(mA.max(x._1, y._1), mB.max(x._2, y._2))
@main def entryPoint(): Unit = {
println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
}
Instead of just printing the pair (20, 3), it fails the compilation with the following error:
12 | println(summon[Max[(Double, Double)]].max((10.0, 3.0), (20.0, -7.0)))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Deferred inline method max in trait Max cannot be invoked
|----------------------------------------------------------------------------
|Inline stack trace
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|This location contains code that was inlined from buggy-example.scala:9
9 | (mA.max(x._1, y._1), mB.max(x._2, y._2))
| ^^^^^^
----------------------------------------------------------------------------
It seems that it should know everything statically. Any clues why it's failing?
If you tried to use alias givens rather than given instances (with-syntax)
inline given maxForDoubles: Max[Double] = new Max[Double]:
inline override def max(a: Double, b: Double): Double = if a < b then b else a
inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] = new Max[(A, B)]:
inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))
this wouldn't work because of
Implementation restriction: nested inline methods are not supported
But with with-syntax nested inline are allowed because
inline given maxForDoubles: Max[Double] with
inline override def max(a: Double, b: Double): Double = if a < b then b else a
inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): Max[(A, B)] with
inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))
is actually
class maxForDoubles extends Max[Double]:
inline override def max(a: Double, b: Double): Double = if a < b then b else a
inline given maxForDoubles: maxForDoubles = new maxForDoubles
class maxForPairs[A, B](using mA: Max[A], mB: Max[B]) extends Max[(A, B)]:
inline override def max(x: (A, B), y: (A, B)): (A, B) = (mA.max(x._1, y._1), mB.max(x._2, y._2))
inline given maxForPairs[A, B](using mA: Max[A], mB: Max[B]): maxForPairs[A, B] = new maxForPairs[A, B]
Indeed, with -Xprint:pickleQuotes option (or -Xprint:typer) the with-syntax produces
// given class maxForDoubles() extends Object(), App.Max[Double] {
// override inline def max(a: Double, b: Double): Double =
// (if a.<(b) then b else a):Double
// }
// final inline given def maxForDoubles: App.maxForDoubles =
// new App.maxForDoubles():App.maxForDoubles
// given class maxForPairs[A >: Nothing <: Any, B >: Nothing <: Any](using
// mA: App.Max[A]
// , mB: App.Max[B]) extends Object(), App.Max[
// Tuple2[maxForPairs.this.A, maxForPairs.this.B]
// ] {
// A
// B
// protected given val mA: App.Max[A]
// protected given val mB: App.Max[B]
// override inline def max(x: Tuple2[maxForPairs.this.A, maxForPairs.this.B]
// ,
// y: Tuple2[maxForPairs.this.A, maxForPairs.this.B]): (A, B) =
// Tuple2.apply[A, B](this.App$maxForPairs$$inline$mA.max(x._1, y._1),
// this.App$maxForPairs$$inline$mB.max(x._2, y._2)
// ):(A, B)
// def App$maxForPairs$$inline$mA: App.Max[A] = maxForPairs.this.mA
// def App$maxForPairs$$inline$mB: App.Max[B] = maxForPairs.this.mB
// }
// final inline given def maxForPairs[A >: Nothing <: Any, B >: Nothing <: Any]
// (
// using mA: App.Max[A], mB: App.Max[B]): App.maxForPairs[A, B] =
// new App.maxForPairs[A, B](using mA, mB)():App.maxForPairs[A, B]
Let's temporarily simplify the with-syntax removing using and maxForDoubles
class maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/ extends Max[(A, B)]:
inline override def max(x: (A, B), y: (A, B)): (A, B) = ??? // (mA.max(x._1, y._1), mB.max(x._2, y._2))
// inline given maxForPairs[A, B]/*(using mA: Max[A], mB: Max[B])*/: maxForPairs[A, B] = new maxForPairs[A, B]
Then the difference between
val inst: maxForPairs[Double, Double] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // compiles
and
val inst: Max[(Double, Double)] = new maxForPairs[Double, Double]
inst.max((10.0, 3.0), (20.0, -7.0)) // doesn't compile: Deferred inline method max in trait Max cannot be invoked
is understandable because there is a rule
Inline methods can also be abstract. An abstract inline method can be implemented only by other inline methods. It cannot be invoked directly:
abstract class A: inline def f: Int object B extends A: inline def f: Int = 22 B.f // OK val a: A = B a.f // error: cannot inline f in A.
https://docs.scala-lang.org/scala3/reference/metaprogramming/inline.html#rules-for-overriding
And max is exactly an abstract inline method in the trait Max.
summon[Max[(Double, Double)]] returns the precise type of implicit: maxForPairs[Double, Double] <: Max[(Double, Double)] (as shapeless.the in Scala 2), not just Max[(Double, Double)] (as implicitly in Scala 2).
But if we restore using and maxForDoubles then for some reason this confuses inlines: both options for val inst (: maxForPairs[Double, Double] and : Max[(Double, Double)]) produce Deferred inline method max in trait Max cannot be invoked.
This seems to be a bug or underspecified feature. Try to open a ticket at https://github.com/lampepfl/dotty/issues
By the way, sometimes with-syntax behaves weirdly in comparison with alias givens (even without inlines):
https://github.com/lampepfl/dotty/issues/8882