Incorrectly renderred signature for parameterless method in error message reporting unimplemented method

[marcinzh]

Compiler version

3.0.0-RC1-bin-20210117-65b17af-NIGHTLY

Minimized code

trait Foo:
  def foo: Int
  def foo2: Int // optionally one more, to trigger Dotty to propose stubs

class Bar extends Foo

Output

class Bar extends Foo
      ^
      class Bar needs to be abstract, since:
      it has 2 unimplemented members.
      /** As seen from class Bar, the missing signatures are as follows.
       *  For convenience, these are usable as stub implementations.
       */
        def foo: => Int = ???
        def foo2: => Int = ???

Expectation

The method signature printed in error message & proposed stub should be:

foo: Int

instead of:

foo: => Int

[wg-romank]

Hi,

I'm quite new to this codebase but I would like to try helping with this one.
Would it be okay?

[wg-romank]

I checked a bit, and so far based on my understanding this could be fixed by handling additional case in PlainPrinter.scala.
Currently when calling showDcl for parameterless method we match onto this case
https://github.com/lampepfl/dotty/blob/b023ec85c7e44dd393394809df6f31e43f3fcf97/compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala#L402
Since parameterless method has info of type ExprType instead of MethodType stubs signatures are not correct.

What I'm not sure about is whether the denotation process should handle parameterless methods differently (to assign them MethodType instead of ExprType) and if there are any other implications due to that.

Please kindly advice on the direction. Thank you.

[smarter]

What I'm not sure about is whether the denotation process should handle parameterless methods differently (to assign them MethodType instead of ExprType) and if there are any other implications due to that.

No, using ExprType makes a difference, def foo: Int is an ExprType whose resultType is a TypeRef of Int whereas def foo(): Int is a MethodType whose parameter list is Nil and resultType is a TypeRef of Int.

Feel free to experiment with changes to the printer to improve the output here, but note that ExprType is also used for by-name parameters (for example in def foo(x: => String): Int, the type of x is an ExprType), so for those the => needs to be preserved in the printed output.

[wg-romank]

Hi @smarter , thank you for the details. I'm sorry I had to pause working on this.
After checking a bit further I would still need some guidance. I hope it is okay.

As you mentioned since ExprType is used for both parameterless methods and by-name parameters it seems it is not possible to pick appropriate presentation based on symbol info value alone. In fact the values look exactly the same:

// def foo: Int
ExprType(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int))

// def foo(x: => Int): Int
MethodType(
  List(...),
  List(
    ExprType(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int))  // <--
  ), 
   ...
)

This structure is lost since toTextRHS is called recursively, but I assume I can somehow get it back (either by using one of the methods taking Context or the Context itself, but I'm a bit lost atm)

Thank you 🙏

[smarter]

You can't really recover this information from Context, instead you need to avoid loosing it somehow, maybe by adding an isTopLevel boolean parameter to toTextRHS and making sure it's set to false when called from paramsText.