-Xfatal-warnings applied to flags passed to scalac itself (inconsistent with Scala 2 behavior)

[arixmkii]

Compiler version

Scala compiler version 3.1.0 -- Copyright 2002-2021, LAMP/EPFL

Minimized example

Calling scalac with -Xfatal-warnings set multiple times results in compilation error

CompilerTests % scalac -version
Scala compiler version 3.1.0 -- Copyright 2002-2021, LAMP/EPFL
CompilerTests % scalac -Xfatal-warnings -Xfatal-warnings some.scala; echo $?
Flag -Werror set repeatedly
1 error found
1

Expectation

It could stay as warning or silently ignored as in Scala 2

CompilerTests % scalac -version
Scala compiler version 2.13.7 -- Copyright 2002-2021, LAMP/EPFL and Lightbend, Inc.
CompilerTests % scalac -Xfatal-warnings -Xfatal-warnings some.scala; echo $?
0

Explanation

If you have sbt build, where multiple plugins are used for setting some compiler settings (like tpolecat and another one private maintained by the company) they may set the same option multiple times. It was safe before, but now such builds will fail if -Xfatal-warnings is used.

There is a workaround like use ~= op in sbt and call distinct over the option set, but it is unsafe for flags, which take value as the next parameter (-encoding utf8 or -release 8), which in sbt will be to consecutive values in the list.

I would agree that it is a project misconfiguration in sbt build, but reporting the issue here still, because it is very inconsistent with Scala 2 behavior.

[arixmkii]

Adding a flag, which will suppress warnings generated specifically from compiler options would be another possible solution.

[som-snytt]

That's a good one. I don't remember if I introduced the warning or only noticed it in passing. ok, I added the warning for "redundantly". They could add a category for compiler config aka options.

➜  ~/projects/dotty/bin/scalac -Werror -d /tmp ok.scala
➜  ~/projects/dotty/bin/scalac -Werror -Werro -d /tmp ok.scala
bad option '-Werro' was ignored
1 error found
➜  ~/projects/dotty/bin/scalac -Werror -Werror -d /tmp ok.scala
Flag -Werror set repeatedly
1 error found
➜  ~/projects/dotty/bin/scalac -Wconf:msg=repeatedly:s -Werror -Werror -d /tmp ok.scala
➜  

[arixmkii]

Reasonable workaround, but to cover all scenarios, there will be plenty of cases. Like implicitConversions flag has different wording and one will need to pass something like -Wconf:msg=redundantly:s, because of the message "Setting -language set to implicitConversions redundantly"

Would be nice to see a category, that covers compiler flags being processed in general.

[som-snytt]

You can write '-Wconf:msg=repeatedly|redundantly:s' where the extra quotes are for shell not sbt.

I'll PR --configuration to work the same as --deprecation. Scala 3 warning categories are currently the few that are tied to options like feature and deprecation. That's a bit heavyweight for miscellaneous categories, but OK (I guess) for "anything related to compiler config".

If I have a problem with my options, -Xlint:config might not help. I like the idea of a plugin like the tpolecat configurator to sanity check options AOT.

[som-snytt]

Still errors! Apparently scala-cli does not present the doubled arg?

➜  snips scalac -d /tmp -deprecation -Werror -Werror i17969.scala
Flag -Werror set repeatedly
No warnings can be incurred under -Werror (or -Xfatal-warnings)
1 warning found
1 error found
➜  snips scalac -version
Scala compiler version 3.7.2 -- Copyright 2002-2025, LAMP/EPFL
➜  snips scala-cli compile --server=false i17969.scala
➜  snips

sbt project errors as expected

➜  i13887 sbt run
[info] welcome to sbt 1.11.4 (Temurin Java 1.8.0_462)
[info] loading project definition from /home/amarki/snips/i13887/project
[info] loading settings for project i13887 from build.sbt...
[info] set current project to i13887 (in build file:/home/amarki/snips/i13887/)
[info] compiling 1 Scala source to /home/amarki/snips/i13887/target/scala-3.7.2/classes ...
[warn] Flag -Werror set repeatedly
[error] No warnings can be incurred under -Werror (or -Xfatal-warnings)
[warn] one warning found
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 3 s, completed Aug 22, 2025 2:28:02 PM
➜  i13887

Forgot to set my JDK back to the current day after testing old scalac versions...