Inlining private method breaks compilation

[scf37]

Compiler version

3.1.0

Minimized code

trait T {
  inline def foo(handler: Int): Unit =
    bar(handler)

  private inline def bar(handler: Int): Unit = ()
}

new T {
  foo(42)
}

Output

undefined: T_this.bar # -1: TermRef(TermRef(NoPrefix,val T_this),bar) at inlining

Expectation

Successful compilation

[odersky]

It's because we do not generate inline accessors for inline methods. See needsAccessor in PrepareInlineable.

I see two possible ways to fix this. Ideally, we do create an inline accessor which is itself defined
inline (and transparent, if the accessed method is transparent). I tried that, but it does not work currently, since the inline accessor does not get inlined.

If that does not work, we could also issue a static error if an inline method calls a private inline method.

[odersky]

It turns out the static error route would invalidate several existing tests and the compiler itself.

[nicolasstucki]

To fix this we first need to fix #14048 as we would probably fit it on the way.

[nicolasstucki]

We should be evaluating this branch but the symbol is NoSymbol.
https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/typer/Inliner.scala#L1584

[nicolasstucki]

The issue comes from transforming new T { foo(42) } into new T { bar(42) }. As bar is private in T it cannot be selected from the anonymous new T.

@odersky is there a way to compute this symbol?

[odersky]

@nicolasstucki Not if it is private.