Report two errors when the same class is defined twice

[olafurpg]

Currently, scalac reports a single error when the same class is compiled twice. Consider the following input:

// foo.scala
object Main
object Main

Running scalac reports

$ scalac foo.scala
foo.scala:2: error: Main is already defined as object Main
object Main
       ^
one error found

I expected scalac to report two errors

foo.scala:2: error: Main is already defined as object Main
object Main
       ^
+ foo.scala:1: error: Main is already defined as object Main
+ object Main
+        ^
- one error found
+ two errors found

This situation where the same class is defined twice can happen frequently in IDEs like Metals where users copy-paste a source file, example scalameta/metals#1027.

[hrhino]

Why? What's wrong with the first definition?

[olafurpg]

Because it’s mostly accidental in what order the files are passed to the compiler and you could argue there is no “first” definition. I may have gone too far with minimization, in the original issue that was reported in Metals the second file was an Emacs generated temporary file “.#Main.scala” which the user did not have open in the editor.

[olafurpg]

In the case when the first definition comes from the classpath during separate compilation then I think the current behavior is working as expected.

[lrytz]

I can see that in an editor it would be useful to have a flag at both locations, but on the command line it would be noise. Maybe the (single) error message could include the position of the other definition?