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maybe warn when "forward reference to a local variable" is shadowing parameter. #14401

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vincenzobaz opened this issue Feb 2, 2022 · 4 comments · May be fixed by #22534
Closed

maybe warn when "forward reference to a local variable" is shadowing parameter. #14401

vincenzobaz opened this issue Feb 2, 2022 · 4 comments · May be fixed by #22534
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area:refchecks area:reporting Error reporting including formatting, implicit suggestions, etc itype:enhancement

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@vincenzobaz
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Compiler version

3.1.1

Minimized code

def f(a: Int) =
  val b = a + 1
  val a = 3

  b

@main def run =
  println(f(4))

Output

➜  dottyforwardbug sbt run
[info] Updated file /home/vincenzo/Desktop/dottyforwardbug/project/build.properties: set sbt.version to 1.5.5
[info] welcome to sbt 1.5.5 (AdoptOpenJDK Java 1.8.0_292)
[info] loading project definition from /home/vincenzo/Desktop/dottyforwardbug/project
[info] loading settings for project dottyforwardbug from build.sbt ...
[info] set current project to dottyforwardbug (in build file:/home/vincenzo/Desktop/dottyforwardbug/)
[info] compiling 1 Scala source to /home/vincenzo/Desktop/dottyforwardbug/target/scala-3.1.1/classes ...
[error] -- [E039] Reference Error: /home/vincenzo/Desktop/dottyforwardbug/src/main/scala/main.scala:2:10 
[error] 2 |  val b = a + 1
[error]   |          ^
[error]   |          a is a forward reference extending over the definition of b
[error] one error found
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 2 s, completed Feb 2, 2022 9:33:55 AM

Expectation

I would expect to receive a 5. Maybe a warning about shadowing.
I do not expect a compilation error.

Notes

I initially thought that this was a problem of top level definitions, so I tried

object Main:
  def f(a: Int) =
    val b = a + 1
    val a = 3

    b

@main def run =
  println(Main.f(4))

obtaining the same error.
@vincenzobaz vincenzobaz added itype:bug stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 2, 2022
@bishabosha bishabosha added area:refchecks and removed stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 2, 2022
@bishabosha
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bishabosha commented Feb 2, 2022 •
edited
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this also fails in the same way in Scala 2, so it might be expected,

i.e. it doesn't matter what the function parameter is called because line 3 defines a:

def f(dkfhsdkj: Int) = {
  val b = a + 1
  val a = 3

  b
}

@som-snytt
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Since last year, Scala 2 tells you where the reference refers to.

scala/scala#9654

The similar example (from 2012) at

scala/bug#12382 (comment)

object Test {
  def main(args: Array[String]) {
    class NotUsed {val xs = args}
    val dummy = false
    // oops, shadows the parameter
    def args = Seq("a","b","c")
  }
}

In solidarity with OP, I too expected a lint for the shadowing. Either I'm remembering a previous speculative conversation about such a lint, or (note to self) just the last tweak to name binding, where an inherited member must not shadow a definition in outer scope.

A linting addendum would be extra: args shadows parameter of main.

This issue also recalls that people expect to have a syntax to refer to a shadowed parameter, similar to selection from this.

@bishabosha bishabosha reopened this Feb 2, 2022
@bishabosha bishabosha added area:reporting Error reporting including formatting, implicit suggestions, etc itype:enhancement and removed itype:bug labels Feb 2, 2022
@bishabosha bishabosha changed the title Shadowing function parameter produces forward reference compilation error maybe warn when "forward reference to a local variable" is shadowing parameter. Feb 2, 2022
@bishabosha
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changed this issue to be about producing warning specifically when there is this forward reference error and the forward reference matches the name of a parameter

@som-snytt som-snytt mentioned this issue Feb 8, 2022
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@odersky
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odersky commented Feb 21, 2022

Shadowing warnings are very hard to get right in all cases. In this case it seems the forward reference error is indication enough of what goes on.

@odersky odersky closed this as completed Feb 21, 2022
@som-snytt som-snytt mentioned this issue Feb 6, 2025
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Forward reference error includes line numbers som-snytt/dotty
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@som-snytt @odersky @vincenzobaz @bishabosha