[backport] Support for Scala 3 syntax under -Xsource:3
#9589
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Leading infix is syntax. |
Just like in Scala 3.0, adding this keyword doesn't change anything, but it will be required in future versions of Scala 3 for non-exhaustive patterns in a for comprehension. We would like to start issuing warnings by default in Scala 3 for code which does not use `case` in those situations, but to not hamper cross-compilation we need Scala 2 to also support that keyword. For details, see: https://dotty.epfl.ch/docs/reference/changed-features/pattern-bindings.html
Like in Scala 3.0, this allows `?` to be used as a type argument in all situations where `_` could be used as a wildcard previously. This should allow us to deprecate the use of `_` as a wildcard in Scala 3 to be able to eventually repurpose it as explained in http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html This is a source-breaking change since a type named `?` is legal in Scala 2 (but not in Scala 3 unless -source 3.0-migration is used). `?` also has a special meaning when the kind-projector plugin is used, but that usage has been deprecated in favor of `*` for a while now.
Instead of:
import foo._
One can now write:
import foo.*
and instead of:
import foo.{bar => baz}
One can now write:
import foo.{bar as baz}
As well as:
import foo.bar as baz
This will let us deprecate the old syntax in a future release of Scala
3 (it's currently only deprecated under `-source future`).
See http://dotty.epfl.ch/docs/reference/changed-features/imports.html
for details but note that unlike Scala 3 this commit does not implement
support for:
import java as j
As that would require deeper changes in the compiler.
Instead of:
foo(s: _*)
One can now write:
foo(s*)
And instead of:
case Seq(elems @ _*) =>
One can now write:
case Seq(elems*) =>
See https://dotty.epfl.ch/docs/reference/changed-features/vararg-splices.html
for details.
Since everything is open and can be used infix by default in Scala 2, these keywords are no-op, but they're useful for cross-compiling with a future version of Scala 3 where they will be required in some cases (with Scala 3.0 they're only required to avoid warnings under `-source future`). See https://dotty.epfl.ch/docs/reference/changed-features/operators.html and http://dotty.epfl.ch/docs/reference/other-new-features/open-classes.html for details.
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This PR backports eee223b from #7660 as well as the following PRs adding support for some Scala 3 syntactic constructs under
-Xsource:3:?as wildcard marker under -Xsource:3 #9560casein pattern bindings under -Xsource:3 #9558openandinfixunder -Xsource:3 #9580However, it does not backport the things that
-Xsource:3does in Scala 2.13 which are not syntactic addition (e.g., deprecations, semantic changes for private case classes, semantic changes for contravariant implicit resolution) as well as the leading infix support from #8419