Make `with` type operator an error by mbovel · Pull Request #26545 · scala/scala3

mbovel · 2026-07-14T14:22:01Z

with as a type operator (A with B) has been deprecated since 3.4. Make it an error from 3.10 by changing MigrationVersion.WithOperator's errorFrom from future to 3.10. It stays a warning in the 3.4-3.9 source versions and a rewriteable migration warning under 3.10-migration.

Migrate the test suite from A with B to A & B (a purely syntactic change: with already desugars to an intersection type via makeAndType) and regenerate the semanticdb and Scala.js check files.

Note: in typed pattern position (case x: T => ...) the type is parsed as a RefinedType, which accepts with but not the & infix operator, so the migrated case x: A with B => must be parenthesised as case x: (A & B) => (matching what the automatic rewrite does).

Have you relied on LLM-based tools in this contribution?

Yes, and I reviewed the output.

How was the solution tested?

Covered by updated existing tests.

`with` as a type operator (`A with B`) has been deprecated since 3.4. Make it an error from 3.10 by changing `MigrationVersion.WithOperator`'s `errorFrom` from `future` to `3.10`. It stays a warning in the 3.4-3.9 source versions and a rewriteable migration warning under 3.10-migration. Migrate the test suite from `A with B` to `A & B` (a purely syntactic change: `with` already desugars to an intersection type via `makeAndType`), regenerate the semanticdb and Scala.js check files, and reorganise the dedicated with-operator tests: - move the 3.4-migration case to tests/warn (it still warns), - drop the future-migration warn test (it is now an error), - add tests/neg/with-type-operator.scala for the default-source error, - point the rewrite test at -source 3.10-migration. In typed pattern position (`case x: T => ...`) the type is parsed as a `RefinedType`, which accepts `with` but not the `&` infix operator, so the migrated `case x: A with B =>` must be parenthesised as `case x: (A & B) =>` (matching what the automatic rewrite does). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mbovel · 2026-07-14T14:22:52Z

Replaced by tests/neg/with-type-operator.scala.

Follow-up to making `with` a type-operator error: migrate the remaining in-repo test inputs that dotty compiles under the default source version. - repl: i6643 (bound), and drop the now-unwritable `with` cases from type-printer/infix (the parallel `&` cases already cover printing). - presentation-compiler: completion / info / inlayHints code snippets. - scripted: scala2-compat/erasure dottyApp (kept scala2Lib on `with`, the two are still erasure-equivalent), source-dependencies/continuations and compilerReporter/simple. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Point the community-build submodules that use the (now removed) `with` type operator at `with-type-operator-error` branches on the dotty-staging forks, where `A with B` is rewritten to `A & B`. Each fix was verified locally by running the corresponding community-build test against the locally published compiler. Verified green: cats, ScalaPB, scalaz, fastparse, scala-parser-combinators, sconfig, libretto, scala-parallel-collections, scala-java8-compat, scala-stm, Lucre, perspective, parboiled2, scala-xml. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

izumi-reflect's fix additionally rewrites `with` inside two embedded test type snippets (assertTypeError / assertCompiles), which a type-level pass can't see. Verified green locally: `izumi-reflect/test` = 177 succeeded, 0 failed at the default source version. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

The Scala.js JUnit test suite is fetched from scala-js (pinned to v$scalaJSVersion) and compiled as-is. Those sources still use the `with` type operator, which is an error since 3.10, breaking `sjsJUnitTests/Test/compileIncremental`. Compile them under 3.9, where `with` is only a (non-fatal, `-Werror` is already off here) warning; 3.9 and 3.10 enable the same language features otherwise. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mbovel · 2026-07-14T20:29:56Z

@sjrd does that seem reasonable to compile sjsJUnitTests with -source:3.9 (59183ed)?

Fixes the three community_build_c failures. `intent` and `pprint` used the `with` type operator only in test sources, which the earlier discovery pass skipped; `upickle` is built by the active `ujson` test even though its own test entry is commented out. All three fixes are verified green locally via their community-build tests. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Fixes the community_build_b failure: cats-effect used the `with` type operator in 7 core files (14 occurrences, including the JS and Native platform sources CI did not reach). The fix compiles cleanly and the full catsEffect3 community-build test passes locally, modulo one load-induced timeout in `DispatcherSuite.complete / cancel race-3` which passes in isolation (111/111). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mbovel · 2026-07-15T09:16:02Z

Prior Slack discussion: https://lampepfl.slack.com/archives/C06FYNL1WDT/p1776758870150489.

SethTisue · 2026-07-15T14:12:21Z

About any community build failures that occur, I wonder how many of them are because the codebase is cross-compiled for both Scala 3 and 2. As of 3.10 we're no longer trying to support that, so perhaps both community builds will need to be slimmed down somewhat.

A middle option would be to try compiling a project with -source:3.9 if it won't compile with default settings, to avoid dropping the project entirely.

SethTisue · 2026-07-15T14:13:38Z

In any case, there was no opposition at core meeting to proceeding with this.

mbovel · 2026-07-16T09:00:18Z

I wonder how many of them are because the codebase is cross-compiled for both Scala 3 and 2.

16/19 updated community build projects cross-compile for Scala 2 and 3.

Claude output

I counted, by inspecting the build configuration of each of the 19 updated submodules:

16 of 19 cross-compile for Scala 2 and 3

Project	Cross versions (from build)
cats	2.12, 2.13, 3
cats-effect-3	2.12, 2.13, 3
scalaz	2.13, 3
ScalaPB	2.12, 2.13, 3 (+ `scala-2`/`scala-3` split dirs)
izumi-reflect	2.12, 2.13, 3 (+ `scala-2*`/`scala-3` split dirs)
upickle	2.12, 2.13, 3 (Mill)
PPrint	2.12, 2.13, 3
fastparse	2.12, 2.13, 3 (shared `src/`)
scala-xml	2.12, 2.13, 3
scala-parser-combinators	2.11, 2.12, 2.13, 3
scala-java8-compat	2.11, 2.12, 2.13, 3
scala-parallel-collections	2.13, 3
scala-stm	2.11, 2.12, 2.13, 3
sconfig	2.11, 2.12, 2.13, 3
parboiled2	2.12, 2.13, 3
Lucre	2.12, 2.13, 3

Scala 3 only (2): libretto and intent — their with is just legacy style from
early Dotty days, not cross-compilation.

Special case (1): perspective — not same-source cross-compiled but a hybrid with
separate scala2/ (macro implementations, 2.13) and dotty/ trees; all 155 of its
with occurrences were in the scala2/ tree, i.e. genuine Scala-2-heritage code.
(The community build force-switches every module to the tested compiler via ++3.x...!,
which is why those files errored and why the & patch verifies green there.)

So 16/19 strictly, 17/19 counting perspective's Scala-2-heritage sources. It also
correlates with volume: the biggest patch counts (scalaz 304 occurrences, cats 197,
izumi-reflect 154, ScalaPB 68) are all cross-built libraries, where A with B was the
only intersection syntax valid under both compilers.

One implication: for these cross-built projects, the & patches on our dotty-staging
fork branches are community-build-only — upstream can't adopt them until they drop
Scala 2 or move those types into version-specific source dirs. That's a real (if mild)
ecosystem-migration cost of erroring in 3.10.

A middle option would be to try compiling a project with -source:3.9 if it won't compile with default settings, to avoid dropping the project entirely.

That sounds good to me; it's the cheap way to make the errors warnings again. In that sense, what we're changing is just the default level of the diagnostic, but users can always pass -source to change it.

mbovel · 2026-07-16T09:02:55Z

 [submodule "community-build/community-projects/ScalaPB"]
 	path = community-build/community-projects/ScalaPB
 	url = https://github.com/dotty-staging/ScalaPB
+	branch = with-type-operator-error


Note: once this PR is validated, we can point each fork's main branch to its pinned fix commit and drop the branch = with-type-operator-error entries from .gitmodules.

jchyb

LGTM! As I understand the comment, this shouldn't be merged until the community build branches are updated (currently I can't even if I wanted to because of the conflict). I was initially concerned about the open community build being usable after this, but now believe it should be fine, since it seems that it will use the latest -source:3.x-migration (at least if I understand VirtusLab/community-build3#576 correctly)

mbovel commented Jul 14, 2026

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mbovel and others added 4 commits July 14, 2026 15:52

mbovel and others added 2 commits July 14, 2026 20:56

mbovel marked this pull request as ready for review July 15, 2026 07:19

mbovel commented Jul 16, 2026

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mbovel requested a review from jchyb July 16, 2026 09:05

mbovel added the needs-squashing PR whose commits should be squashed by the author or via the "Squash and Merge" button label Jul 16, 2026

Gedochao assigned jchyb Jul 21, 2026

jchyb approved these changes Jul 21, 2026

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Make `with` type operator an error#26545

Make `with` type operator an error#26545
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mbovel commented Jul 16, 2026

16 of 19 cross-compile for Scala 2 and 3

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mbovel commented Jul 14, 2026

Have you relied on LLM-based tools in this contribution?

How was the solution tested?

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mbovel commented Jul 14, 2026

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mbovel commented Jul 16, 2026

16 of 19 cross-compile for Scala 2 and 3

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SethTisue commented Jul 15, 2026 •

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