Inline and beta reduction can generate incorrect `Inlined` trees

[nicolasstucki]

Compiler version

3.2.1

Minimized code

Compiling with -Ycheck:all

def method(using String): String = ???

inline def inlineMethod(inline op: String => Unit)(using String): Unit =
  println(op(method))

def test(using String) =
  inlineMethod(c => ())

Output

*** error while checking test.scala after phase inlining ***
java.util.NoSuchElementException: head of empty list while running Ycheck on tests.scala
exception occurred while compiling tests.scala
java.util.NoSuchElementException: head of empty list while compiling tests.scala
Exception in thread "main" java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:662)
        at scala.collection.immutable.Nil$.head(List.scala:661)
        at dotty.tools.dotc.transform.YCheckPositions$$anon$1.traverse(YCheckPositions.scala:45)
        ...

The tree is

def test(using x$1: String): Unit =
      {{ /* inlined from 
        inlineMethod(
          {
            def $anonfun(c: String): Unit = ()
            closure($anonfun)
          }
        )(x$1)
       */
        println(
          {{ /* inlined from outside */
            {
              val c$proxy1: String = method({{ /* inlined from outside */x$1}})
              ()
            }
          }}
        ):Unit
      }}

It seems we are adding an Inlined with no call when beta-reducing the lambda.

Expectation

It should work. Beta reduction should not add this additional Inlined node, or it should balance it. Maybe the proxy should be in the Inlined bindings.

[nicolasstucki]

The issue is that we beta reduce

    def test(using x$1: String): Unit =
      {{/* inlined from 
        inlineMethod(
          {
            def $anonfun(c: String): Unit = ()
            closure($anonfun)
          }
        )(x$1)
       */
        println(
          {{/* inlined from outside */
            {
              def $anonfun(c: String): Unit = ()
              closure($anonfun)
            }
          }}.apply(method({{/* inlined from outside */x$1}}))
        ):Unit
      }}

to

 def test(using x$1: String): Unit =
      {{/* inlined from 
        inlineMethod(
          {
            def $anonfun(c: String): Unit = ()
            closure($anonfun)
          }
        )(x$1)
       */
        println(
          {{/* inlined from outside */
            {
              val c$proxy1: String = method({{/* inlined from outside */x$1}})
              ()
            }
          }}
        ):Unit
      }}

It looks like what we need is to generate the binding outside the Inlined tree

 def test(using x$1: String): Unit =
      {{/* inlined from 
        inlineMethod(
          {
            def $anonfun(c: String): Unit = ()
            closure($anonfun)
          }
        )(x$1)
       */
        println({
          val c$proxy1: String = method({{/* inlined from outside */x$1}})
          {{/* inlined from outside */ () }}
        }
        ):Unit
      }}

[nicolasstucki]

A similar case that fails in a different place

def method(using String): String = ???

inline def identity[T](inline x: T): T = x

inline def inlineMethod(inline op: String => Unit)(using String): Unit =
  println(identity(op)(method))

def test(using String) =
  inlineMethod(c => print(c))