Don't allow inherited names to shadow names defined in the outer scope

[odersky]

See scala/bug#11921 (comment)

I think it's worth experimenting with a rule which says that an inherited member is not allowed to shadow a definition of the same name in an outer scope. Imports in outer scopes would be unaffected.

This could be a warning in 3.0 and an error in 3.1

[odersky]

Example:

class C:
  val x = 2
object a:
  val x = 1
  class D extends C:
    println(x)

Here we currently print 2. Instead, I would like to see an error or warning like this:

Reference to `x` is ambiguous
It is both defined in object a
and inherited subsequently by class D

Importantly, I would flag it as an error only if x is referenced as a simple name. Writing this.x is OK. And using x not at all in D is also OK.

[dwijnand]

What if they're split files?

// C.scala
class C:
  val x = 2

// a.scala
object a:
  val x = 1
  class D extends C:
    println(x)

Should x shadow C's x cleanly there, no warnings nor errors?

Just for the record, I, personally, dislike differences in behaviour based on same-file vs split files (like for imports/scope, and the empty package iirc), but I wanted to mention it to avoid the differences being inconsistent.

[dwijnand]

To answer my own question, no it doesn't matter (which I think is inconsistent?):

$ dotc $(f "class C { val x = 2 }; object a { val x = 1; class D extends C { println(x) } }")
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.My0o23jD.scala
-- [E049] Reference Error: /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.My0o23jD.scala:1:73
1 |class C { val x = 2 }; object a { val x = 1; class D extends C { println(x) } }
  |                                                                         ^
  |                                     Reference to x is ambiguous,
  |                                     it is both defined in object a
  |                                     and inherited subsequently in class D

longer explanation available when compiling with `-explain`
1 error found

$ dotc $(f "package p; class C { val x = 2 }; object a { val x = 1; class D extends C { println(x) } }")
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.ig9YONiu.scala
-- [E049] Reference Error: /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.ig9YONiu.scala:1:84
1 |package p; class C { val x = 2 }; object a { val x = 1; class D extends C { println(x) } }
  |                                                                                    ^
  |                                     Reference to x is ambiguous,
  |                                     it is both defined in object a
  |                                     and inherited subsequently in class D

longer explanation available when compiling with `-explain`
1 error found

$ dotc $(f "class C { val x = 2 }") $(f "object a { val x = 1; class D extends C { println(x) } }")
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.69wrO4Fc.scala
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.LFHSk5nu.scala
-- [E049] Reference Error: /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.LFHSk5nu.scala:1:50
1 |object a { val x = 1; class D extends C { println(x) } }
  |                                                  ^
  |                                     Reference to x is ambiguous,
  |                                     it is both defined in object a
  |                                     and inherited subsequently in class D

longer explanation available when compiling with `-explain`
1 error found

$ dotc $(f "package p; class C { val x = 2 }") $(f "package p; object a { val x = 1; class D extends C { println(x) } }")
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.YA2xd2pD.scala
Wrote /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.2DE5jCMH.scala
-- [E049] Reference Error: /var/folders/yh/qsn4cxqj47gbhh9_prlxqy3r0000gn/T/scala-test.2DE5jCMH.scala:1:61
1 |package p; object a { val x = 1; class D extends C { println(x) } }
  |                                                             ^
  |                                     Reference to x is ambiguous,
  |                                     it is both defined in object a
  |                                     and inherited subsequently in class D

longer explanation available when compiling with `-explain`
1 error found

$ dotc -version
Dotty compiler version 0.24.0-bin-SNAPSHOT-git-3e888f2 -- Copyright 2002-2020, LAMP/EPFL