Regression for `inline if` when condition is defined in trait

[WojciechMazur]

Based on Open CB failure for zio/izumi-reflect Open CB #37007

Bisect points to 342ebf3

Compiler version

3.3.0-RC1-bin-20230105-b65b0f2-NIGHTLY

Minimized code

trait Logger {
  inline def debug: debug = valueOf[debug]
  final type debug = false

  // fails
  inline final def log(inline s: String): Unit = {
    inline if (debug) println(" -> " + s)
  }
}

trait BaseLogger extends Logger {
  // fails
  def bar() = log("case1")
}

object Logger {
  // works
  inline def log(inline shift: Int, s: String): Unit = {
    inline if (valueOf[Logger#debug]) println(" " * shift + " -> " + s)
  }
}

@main def Test = {
  val x: BaseLogger = ???
  // fails
  x.log("case2")
  Logger.log(2, "bar")
}

Output

-- Error: /workspace/bisect/test.scala:13:17 -----------------------------------
13 |  def bar() = log("case1")
   |              ^^^^^^^^^^^^
   |Cannot reduce `inline if` because its condition is not a constant value: {
   |  val Logger_this: (Logger_this² : (BaseLogger.this : BaseLogger)) =
   |    Logger_this²
   |  {
   |    given val ev: ValueOf[(false : Boolean)] =
   |      new ValueOf[(false : Boolean)](false)
   |    false
   |  }:Logger_this.debug:Logger_this.debug
   |}
   |
   |where:    Logger_this  is a value in method bar
   |          Logger_this² is a value in method bar
   |----------------------------------------------------------------------------
   |Inline stack trace
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |This location contains code that was inlined from test.scala:7
 7 |    inline if (debug) println(" -> " + s)
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ----------------------------------------------------------------------------

Expectation

Should compile

[SethTisue]

The longer reproducer is also good to have, but here's one that's absolutely minimal afaict:

trait Logger:
  inline def debug: False = false
  type False = false
  inline def log(): Unit =
    inline if debug then ()

class Test:
  def test(l: Logger) = l.log()

[error] -- Error: /Users/tisue/tmp/20230110/S.scala:8:29 -------------------------------
[error] 8 |  def test(l: Logger) = l.log()
[error]   |                        ^^^^^^^
[error]   |Cannot reduce `inline if` because its condition is not a constant value: {
[error]   |  val Logger_this: (Logger_this² : (l : Logger)) = Logger_this²
[error]   |  false:Logger_this.False
[error]   |}
[error]   |
[error]   |where:    Logger_this  is a value in method test
[error]   |          Logger_this² is a value in method test
[error]   |-----------------------------------------------------------------------------
[error]   |Inline stack trace
[error]   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]   |This location contains code that was inlined from S.scala:5
[error] 5 |    inline if debug then ()
[error]   |    ^^^^^^^^^^^^^^^^^^^^^^^
[error]    -----------------------------------------------------------------------------
[error] one error found

[SethTisue]

Dale noticed that the inliner is looking for a ConstantValue tree, but instead it sees the following block, which has been inserted as part of this handling:

{
  val Logger_this: (Logger_this² : (l : Logger)) = Logger_this²
  false:Logger_this.False
}

it has the right type but not the expected tree shape. Perhaps the inliner shouldn't be so picky about the tree shape? Or would that open the barn door too wide?

[SethTisue]

Dale found #10107 (because one of our experiments made it fail) which is relevant  — note the exchange between Jamie and Nico about whether looking inside true: Boolean is fair game or not, in the absence of transparent.