Private class constructor is public at the bytecode level

[julienrf]

Classes with private constructors are actually public at the bytecode level, this creates two issues:

• it can be called from Java code
• MiMa reports error if its signature changes

Compiler version

3.2.1

Minimized code

// Foo.scala
package bug

class Foo private (x: Int)

object Foo:
  def apply(x: Int) = new Foo(x)

// Bar.java
package bug

class Bar {
  public void bar() {
    Foo foo = new Foo(42);
  }
}

Output

The code compiles.

Expectation

The Java compilation should fail.

Possible solution

In a discussion, @smarter suggested to emit the constructor as ACC_SYNTHETIC to make it non-accessible from Java, and ignored by mima (see lightbend-labs/mima#92). Note that that change would still be binary compatible with possible existing Java code that would call such constructors (but it would be source incompatible).

Related Scala 2 issue: scala/bug#12711

[julienrf]

After some discussion in scala/bug#12711, we’ve concluded that MiMa should handle that, not the compiler. Follow-up issue is lightbend-labs/mima#738.