[backport] Underscore changes for -Xsource:3: parse +_/-_ and prefix wildcard variables introduced by ?-syntax with ?
#9620
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…eholder syntax This change allows `kind-projector` plugin to rewrite `+_` and `-_` tokens to type lambdas, in line with proposed syntax for Scala 3.2 in http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html When used in conjunction with `-Xsource:3` this will let the user use `?` for wildcards and `_` for placeholders, letting the user cross-compile the same sources with Scala 3 with `-source:3.2` flag. This change is not source breaking since currently `+_` and `-_` fail to parse entirely, this change also does not allow the user to declare types with these names without backticks, they can only be used as part of a type tree. Gate `-_`/`+_` parsing behind `-Xsource:3` to guarantee minimal disruption to existing code
This change names wildcards written with Scala 3 `?` syntax with `?$N` pattern instead of `_$N` used for Scala 2 wildcards There are two reasons for it: - To allow `kind-projector` to implement Scala 3 underscore syntax for type lambdas by transforming old-style underscores, but leaving Scala 3 underscores intact - To show a mildly more relevant error message, since a wildcard introduced by `?` will now also have a name with `?` in the error message
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lrytz
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May 10, 2021
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This PR backports the following changes to Scala 2.12:
+_and-_in types as identifiers under-Xsource:3to support Scala 3.2 placeholder syntax #9605