Support variance annotations in -Ykind-projector mode

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -660,6 +660,9 @@ object StdNames {
       final val STAR : N  = "*"
       final val TILDE: N  = "~"
 
+      final val MINUS_STAR: N = "-*"
+      final val PLUS_STAR : N = "+*"
+
       final val isUnary: Set[Name] = Set(MINUS, PLUS, TILDE, BANG)
     }
 

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1447,8 +1447,15 @@ object Parsers {
       }
     }
 
-    private def makeKindProjectorTypeDef(name: TypeName): TypeDef =
-      TypeDef(name, WildcardTypeBoundsTree()).withFlags(Param)
+    private def makeKindProjectorTypeDef(name: TypeName): TypeDef = {
+      val isVarianceAnnotated = name.startsWith("+") || name.startsWith("-")
+      // We remove the variance marker from the name without passing along the specified variance at all
+      // The real variance will be inferred at a later stage but may contradict the variance specified,
+      // This is ok, because `-Ykind-projector` is for cross-compiling existing Scala 2 code, not for writing new code,
+      // we may assume that variance annotations have already been checked by the Scala 2 compiler.
+      val unannotatedName = if (isVarianceAnnotated) name.mapLast(_.drop(1)) else name
+      TypeDef(unannotatedName, WildcardTypeBoundsTree()).withFlags(Param)
+    }
 
     /** Replaces kind-projector's `*` in a list of types arguments with synthetic names,
      *  returning the new argument list and the synthetic type definitions.
@@ -1457,7 +1464,7 @@ object Parsers {
       val tparams = new ListBuffer[TypeDef]
 
       val newParams = params.mapConserve {
-        case param @ Ident(tpnme.raw.STAR) =>
+        case param @ Ident(tpnme.raw.STAR | tpnme.raw.MINUS_STAR | tpnme.raw.PLUS_STAR) =>
           val name = WildcardParamName.fresh().toTypeName
           tparams += makeKindProjectorTypeDef(name)
           Ident(name)

tests/pos-special/kind-projector.scala

@@ -4,6 +4,14 @@ trait Foo[F[_]]
 trait Qux[F[_, _]]
 trait Baz[F[_], A, B]
 
+trait FooPlus[+F[+_]]
+trait QuxPlus[+F[+_, +_]]
+trait BazPlus[+F[+_], +A, +B]
+
+trait FooMinus[-F[-_]]
+trait QuxMinus[-F[-_, -_]]
+trait BazMinus[-F[-_], -A, -B]
+
 class Bar1 extends Foo[Either[Int, *]]
 class Bar2 extends Foo[Either[*, Int]]
 class Bar3 extends Foo[* => Int]
@@ -13,3 +21,23 @@ class Bar6 extends Foo[λ[x => Either[Int, x]]]
 class Bar7 extends Qux[λ[(x, y) => Either[y, x]]]
 class Bar8 extends Foo[Baz[Int => *, *, Int]]
 class Bar9 extends Foo[λ[x => Baz[x => *, Int, x]]]
+
+class BarPlus1 extends FooPlus[Either[Int, +*]]
+class BarPlus2 extends FooPlus[Either[+*, Int]]
+class BarPlus3 extends FooPlus[Int => +*]
+class BarPlus4 extends FooPlus[(Int, +*, Int)]
+class BarPlus5 extends FooPlus[λ[`+x` => Either[Int, x]]]
+class BarPlus6 extends QuxPlus[λ[(`+x`, `+y`) => Either[y, x]]]
+class BarPlus7 extends FooPlus[BazPlus[Int => +*, +*, Int]]
+
+class BarMinus1 extends FooMinus[-* => Int]
+
+class VarianceAnnotationIsActuallyIgnored1 extends FooPlus[Either[Int, -*]]
+class VarianceAnnotationIsActuallyIgnored2 extends FooPlus[Either[-*, Int]]
+class VarianceAnnotationIsActuallyIgnored3 extends FooMinus[+* => Int]
+class VarianceAnnotationIsActuallyIgnored4 extends FooPlus[Int => -*]
+class VarianceAnnotationIsActuallyIgnored5 extends FooPlus[(Int, -*, Int)]
+class VarianceAnnotationIsActuallyIgnored6 extends FooPlus[λ[`-x` => Either[Int, x]]]
+class VarianceAnnotationIsActuallyIgnored7 extends QuxPlus[λ[(`-x`, `-y`) => Either[y, x]]]
+class VarianceAnnotationIsActuallyIgnored8 extends FooPlus[BazPlus[Int => -*, -*, Int]]
+class VarianceAnnotationIsActuallyIgnored9 extends Foo[λ[`-x` => BazPlus[x => -*, Int, x]]]