Transparent inline method call doesn't expand inside another inline method

Compiler version

3.0.0

Minimized code

First sample

transparent inline def transform(inline a: Any): Any = inline a match {
  case x: Byte   => x
  case x: Short  => x
  case x: Int    => x
  case x: Long   => x
  case x: Float  => x
  case x: Double => x
  case _         => a
}

@main def test = {
  println(transform(0) < 5)
}

Second sample

transparent inline def transform(inline a: Any): Any = inline a match {
  case x: Byte   => x
  case x: Short  => x
  case x: Int    => x
  case x: Long   => x
  case x: Float  => x
  case x: Double => x
  case _         => a
}

inline def lt(inline a: Any, inline b: Double): Boolean = transform(a) < b

@main def test = {
  println(lt(0, 5))
}

Expectation

The first sample compiles as expected.

The second sample doesn’t compile but should. In lt, the transform call is fully inlined so I assume the compiler should be able to find the right return type.

[nicolasstucki]

We describe the semantics of inlining in more detail in http://biboudis.github.io/papers/inlining-scala20.pdf

[odersky]

Calls are never inlined in inline methods, so this does not work.

But I agree that the different phases of inlining can be confusing. The question is whether we can and should do all of inlining in typer again to avoid these surprises.