Use infix extension to indicate right associativity with natural order

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -996,11 +996,18 @@ object desugar {
 
               def badRightAssoc(problem: String) =
                 report.error(em"right-associative extension method $problem", mdef.srcPos)
-                () // extParamss ++ mdef.paramss
+                extParamss ++ mdef.paramss
 
               rightParam match
                 case ValDefs(vparam :: Nil) =>
-                  if !vparam.mods.is(Given) then
+                  if vparam.mods.is(Given) then
+                    badRightAssoc("cannot start with using clause")
+                  else if mdef.mods.is(Infix) then
+                    // New encoding:
+                    // we keep the extension method as is and rely on the swap of arguments at call site
+                    extParamss ++ mdef.paramss
+                  else
+                    // Old encoding:
                     // we merge the extension parameters with the method parameters,
                     // swapping the operator arguments:
                     // e.g.
@@ -1010,16 +1017,13 @@ object desugar {
                     //   def %:[A](using B)[E](f: F)(c: C)(using D)(g: G)(using H): Res = ???
                     //
                     // If you change the names of the clauses below, also change them in right-associative-extension-methods.md
-                    // val (leftTyParamsAndLeadingUsing, leftParamAndTrailingUsing) = extParamss.span(isUsingOrTypeParamClause)
-                    () // leftTyParamsAndLeadingUsing ::: rightTyParams ::: rightParam :: leftParamAndTrailingUsing ::: paramss1
-                  else
-                    badRightAssoc("cannot start with using clause")
+                    val (leftTyParamsAndLeadingUsing, leftParamAndTrailingUsing) = extParamss.span(isUsingOrTypeParamClause)
+                    leftTyParamsAndLeadingUsing ::: rightTyParams ::: rightParam :: leftParamAndTrailingUsing ::: paramss1
                 case _ =>
                   badRightAssoc("must start with a single parameter")
             case _ =>
               // no value parameters, so not an infix operator.
-              () // extParamss ++ mdef.paramss
-          extParamss ++ mdef.paramss
+              extParamss ++ mdef.paramss
         else
           extParamss ++ mdef.paramss
     ).withMods(mdef.mods | ExtensionMethod)

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -7,7 +7,7 @@ import util.{SourceFile, SourcePosition, SrcPos}
 import core.Contexts.*
 import core.Decorators.*
 import core.NameOps.*
-import core.Flags.{JavaDefined, ExtensionMethod}
+import core.Flags.{JavaDefined, ExtensionMethod, Infix}
 import core.StdNames.nme
 import ast.Trees.mods
 import annotation.constructorOnly
@@ -215,8 +215,8 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
         check(tree.trailingParamss)
       case tree: DefDef if tree.mods.is(ExtensionMethod) =>
         tree.paramss match
-          // case vparams1 :: vparams2 :: rest if tree.name.isRightAssocOperatorName =>
-          //   // omit check for right-associatiove extension methods; their parameters were swapped
+          case vparams1 :: vparams2 :: rest if tree.name.isRightAssocOperatorName && !tree.mods.is(Infix) =>
+            // omit check for right-associatiove extension methods; their parameters were swapped
           case _ =>
             check(tree.paramss)
         check(tree.tpt)

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -933,35 +933,35 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         val coreSig =
           if isExtension then
             val paramss =
-              // if tree.name.isRightAssocOperatorName then
-              //   // If you change the names of the clauses below, also change them in right-associative-extension-methods.md
-              //   // we have the following encoding of tree.paramss:
-              //   //   (leftTyParams ++ leadingUsing
-              //   //      ++ rightTyParams ++ rightParam
-              //   //      ++ leftParam ++ trailingUsing ++ rest)
-              //   //   e.g.
-              //   //     extension [A](using B)(c: C)(using D)
-              //   //       def %:[E](f: F)(g: G)(using H): Res = ???
-              //   //   will have the following values:
-              //   //   - leftTyParams = List(`[A]`)
-              //   //   - leadingUsing = List(`(using B)`)
-              //   //   - rightTyParams = List(`[E]`)
-              //   //   - rightParam = List(`(f: F)`)
-              //   //   - leftParam = List(`(c: C)`)
-              //   //   - trailingUsing = List(`(using D)`)
-              //   //   - rest = List(`(g: G)`, `(using H)`)
-              //   // we need to swap (rightTyParams ++ rightParam) with (leftParam ++ trailingUsing)
-              //   val (leftTyParams, rest1) = tree.paramss.span(isTypeParamClause)
-              //   val (leadingUsing, rest2) = rest1.span(isUsingClause)
-              //   val (rightTyParams, rest3) = rest2.span(isTypeParamClause)
-              //   val (rightParam, rest4) = rest3.splitAt(1)
-              //   val (leftParam, rest5) = rest4.splitAt(1)
-              //   val (trailingUsing, rest6) = rest5.span(isUsingClause)
-              //   if leftParam.nonEmpty then
-              //     leftTyParams ::: leadingUsing ::: leftParam ::: trailingUsing ::: rightTyParams ::: rightParam ::: rest6
-              //   else
-              //     tree.paramss // it wasn't a binary operator, after all.
-              // else
+              if tree.name.isRightAssocOperatorName && !tree.mods.is(Infix) && !tree.symbol.is(Infix) then
+                // If you change the names of the clauses below, also change them in right-associative-extension-methods.md
+                // we have the following encoding of tree.paramss:
+                //   (leftTyParams ++ leadingUsing
+                //      ++ rightTyParams ++ rightParam
+                //      ++ leftParam ++ trailingUsing ++ rest)
+                //   e.g.
+                //     extension [A](using B)(c: C)(using D)
+                //       def %:[E](f: F)(g: G)(using H): Res = ???
+                //   will have the following values:
+                //   - leftTyParams = List(`[A]`)
+                //   - leadingUsing = List(`(using B)`)
+                //   - rightTyParams = List(`[E]`)
+                //   - rightParam = List(`(f: F)`)
+                //   - leftParam = List(`(c: C)`)
+                //   - trailingUsing = List(`(using D)`)
+                //   - rest = List(`(g: G)`, `(using H)`)
+                // we need to swap (rightTyParams ++ rightParam) with (leftParam ++ trailingUsing)
+                val (leftTyParams, rest1) = tree.paramss.span(isTypeParamClause)
+                val (leadingUsing, rest2) = rest1.span(isUsingClause)
+                val (rightTyParams, rest3) = rest2.span(isTypeParamClause)
+                val (rightParam, rest4) = rest3.splitAt(1)
+                val (leftParam, rest5) = rest4.splitAt(1)
+                val (trailingUsing, rest6) = rest5.span(isUsingClause)
+                if leftParam.nonEmpty then
+                  leftTyParams ::: leadingUsing ::: leftParam ::: trailingUsing ::: rightTyParams ::: rightParam ::: rest6
+                else
+                  tree.paramss // it wasn't a binary operator, after all.
+              else
                 tree.paramss
             val trailingParamss = paramss
               .dropWhile(isUsingOrTypeParamClause)

library/src/scala/IArray.scala

@@ -319,10 +319,14 @@ object IArray:
     def zipAll[T1 >: T, U](that: Iterable[U], thisElem: T1, thatElem: U): IArray[(T1, U)] = genericArrayOps(arr).zipAll(that, thisElem, thatElem)
     def zipWithIndex: IArray[(T, Int)] = genericArrayOps(arr).zipWithIndex
 
-  extension [T, U >: T: ClassTag](arr: IArray[U])
-    def ++:(prefix: IterableOnce[T]): IArray[U] = genericArrayOps(arr).prependedAll(prefix)
-    def ++:(prefix: IArray[T]): IArray[U] = genericArrayOps(arr).prependedAll(prefix)
-    def +:(x: T): IArray[U] = genericArrayOps(arr).prepended(x)
+  extension [T, U >: T: ClassTag](prefix: IterableOnce[T])
+    def ++:(arr: IArray[U]): IArray[U] = genericArrayOps(arr).prependedAll(prefix)
+
+  extension [T, U >: T: ClassTag](prefix: IArray[T])
+    def ++:(arr: IArray[U]): IArray[U] = genericArrayOps(arr).prependedAll(prefix)
+
+  extension [T, U >: T: ClassTag](x: T)
+    def +:(arr: IArray[U]): IArray[U] = genericArrayOps(arr).prepended(x)
 
   // For backwards compatibility with code compiled without -Yexplicit-nulls
   private inline def mapNull[A, B](a: A, inline f: B): B =

presentation-compiler/test/dotty/tools/pc/tests/hover/HoverTypeSuite.scala

@@ -152,8 +152,8 @@ class HoverTypeSuite extends BaseHoverSuite:
          |class C
          |
          |object Foo:
-         |    extension [R](using A)(res: R)(using B)
-         |      def %:[T](main: T)(using C): R = ???
+         |    extension [T](using A)(main: T)(using B)
+         |      def %:[R](res: R)(using C): R = ???
          |    given A with {}
          |    given B with {}
          |    given C with {}
@@ -162,7 +162,7 @@ class HoverTypeSuite extends BaseHoverSuite:
          |end Foo
          |""".stripMargin,
       """|Int
-         |extension [R](using A)(using B)(res: R) def %:[T](main: T)(using C): R""".stripMargin.hover
+         |extension [T](using A)(main: T) def %:[R](res: R)(using B)(using C): R""".stripMargin.hover
     )
 
   @Test def `using` =

tests/neg-custom-args/captures/lazylists-exceptions.scala

@@ -20,8 +20,8 @@ final class LazyCons[+T](val x: T, val xs: () => LazyList[T]^) extends LazyList[
   def tail: LazyList[T]^{this} = xs()
 end LazyCons
 
-extension [A](xs1: => LazyList[A]^)
-  def #:(x: A): LazyList[A]^{xs1} =
+extension [A](x: A)
+  def #:(xs1: => LazyList[A]^): LazyList[A]^{xs1} =
     LazyCons(x, () => xs1)
 
 def tabulate[A](n: Int)(gen: Int => A): LazyList[A]^{gen} =

tests/neg/i13075.scala

@@ -8,8 +8,8 @@ object Implementing_Tuples:
   type *:[H, T <: Tup] = ConsTup[H, T] // for type matching
   type EmptyTup = EmptyTup.type        // for type matching
 
-  extension [T <: Tup](tail: T)
-    def *:[H](head: H) = ConsTup(head, tail)
+  extension [H](head: H)
+    def *:[T <: Tup](tail: T) = ConsTup(head, tail)
 
   type Fold[T <: Tup, Seed, F[_,_]] = T match
     case EmptyTup => Seed

tests/neg/i9562.scala

@@ -9,5 +9,4 @@ object Unrelated:
     def h1: Int = foo     // error
     def h2: Int = h1 + 1  // OK
     def h3: Int = g       // error
-  extension (x: Int)
-    def ++:(f: Foo): Int = f.h1 + x  // OK
+    def ++: (x: Int): Int = h1 + x  // OK

tests/pos-custom-args/captures/lazylists-exceptions.scala

@@ -42,8 +42,8 @@ extension [A](xs: LzyList[A]^)
     if n == 0 then xs else xs.tail.drop(n - 1)
 end extension
 
-extension [A](xs1: => LzyList[A]^)
-  def #:(x: A): LzyList[A]^{xs1} =
+extension [A](x: A)
+  def #:(xs1: => LzyList[A]^): LzyList[A]^{xs1} =
     LzyCons(x, () => xs1)
 
 def lazyCons[A](x: A, xs1: => LzyList[A]^): LzyList[A]^{xs1} =

tests/pos-custom-args/captures/logger.scala

@@ -32,8 +32,8 @@ final class LazyCons[+T](val x: T, val xs: () => LazyList[T]^) extends LazyList[
   def tail: LazyList[T]^{this} = xs()
 end LazyCons
 
-extension [A](xs1: => LazyList[A]^)
-  def #::(x: A): LazyList[A]^{xs1} =
+extension [A](x: A)
+  def #::(xs1: => LazyList[A]^): LazyList[A]^{xs1} =
     LazyCons(x, () => xs1)
 
 extension [A](xs: LazyList[A]^)

tests/pos-custom-args/captures/strictlists.scala

@@ -28,11 +28,12 @@ extension [A](xs: StrictList[A])
   def concat(ys: StrictList[A]): StrictList[A] =
     if xs.isEmpty then ys
     else xs.head #: xs.tail.concat(ys)
-
-  def #:(x: A): StrictList[A] =
-    StrictCons(x, xs)
 end extension
 
+extension [A](x: A)
+  def #:(xs1: StrictList[A]): StrictList[A] =
+    StrictCons(x, xs1)
+
 def tabulate[A](n: Int)(gen: Int => A) =
   def recur(i: Int): StrictList[A] =
     if i == n then StrictNil

tests/pos/i19197.scala

@@ -1,5 +1,5 @@
 extension (tuple: Tuple)
-  def **:[T >: tuple.type <: Tuple, H](x: H): H *: T = ???
+  infix def **:[T >: tuple.type <: Tuple, H](x: H): H *: T = ???
 
 def test1: (Int, String, Char) = 1 **: ("a", 'b')
 def test2: (Int, String, Char) = ("a", 'b').**:(1)

tests/pos/i9562.scala

@@ -8,6 +8,4 @@ object Unrelated:
   extension (f: Foo)
     def h1: Int = 0
     def h2: Int = h1 + 1  // OK
-
-  extension (x: Int)
-    def ++: (f: Foo): Int = f.h2 + x  // OK
+    def ++: (x: Int): Int = h2 + x  // OK

tests/pos/reference/extension-methods.scala

@@ -10,7 +10,7 @@ object ExtMethods:
   assert(circle.circumference == circumference(circle))
 
   extension (x: String) def < (y: String) = x.compareTo(y) < 0
-  extension [Elem](xs: Seq[Elem]) def #: (x: Elem) = x +: xs
+  extension [Elem](x: Elem) def #: (xs: Seq[Elem]) = x +: xs
   extension (x: Number) infix def min (y: Number) = x
 
   assert("a" < "bb")

tests/run/errorhandling/Result.scala

@@ -35,15 +35,12 @@ object Result:
       case (Err(e), Ok(_))    => Err(e :: Nil)
       case (Err(e1), Err(e2)) => Err(e1 :: e2 :: Nil)
 
-  end extension
-
-  extension [U <: Tuple, E](other: Result[U, List[E]])
     /** Validate both `r` and `other`; return a tuple of successes or a list of failures.
      *  Unlike with `zip`, the right hand side `other` must be a `Result` returning a `Tuple`,
      *  and the left hand side is added to it. See `Result.empty` for a convenient
      *  right unit of chains of `*:`s.
      */
-    def *: [T](r: Result[T, E]): Result[T *: U, List[E]] = (r, other) match
+    def *: [U <: Tuple](other: Result[U, List[E]]): Result[T *: U, List[E]] = (r, other) match
       case (Ok(x), Ok(ys))     => Ok(x *: ys)
       case (Ok(_), es: Err[?]) => es
       case (Err(e), Ok(_))     => Err(e :: Nil)

tests/run/export-in-extension.scala

@@ -11,7 +11,7 @@ object O:
     export cm.*
     def succ: Int = x + 1
     def succ2: Int = succ + 1
-    def ::: (y: Int) = y - x
+    def ::: (y: Int) = x - y
 
 object O2:
   import O.C
@@ -20,7 +20,7 @@ object O2:
     export cm.{bar, baz, bam, ::}
     def succ: Int = x + 1
     def succ2: Int = succ + 1
-    def ::: (y: Int) = y - x
+    def ::: (y: Int) = x - y
 
 @main def Test =
   import O.*

tests/run/i11583.scala

@@ -12,11 +12,11 @@ class Env:
 //     */
 //   def &&:[T <: ctx.Term](trm: T)(ext: env.Extra): (ctx.Type, T, env.Extra) = (tpe, trm, ext)
 
-extension [Ctx <: Context, T <: Boolean](using ctx: Ctx)(trm: T)(using env: Env)
-  def :#:(tpe: String)(ext: env.Extra): (String, T, env.Extra) = (tpe, trm, ext)
+extension [Ctx <: Context](using ctx: Ctx)(tpe: String)(using env: Env)
+  def :#:[T <: Boolean](trm: T)(ext: env.Extra): (String, T, env.Extra) = (tpe, trm, ext)
 
-extension [T <: Tuple](t: T)
-  def :*:[A](a: A): A *: T = a *: t
+extension [A](a: A)
+  def :*:[T <: Tuple](t: T): A *: T = a *: t
 
 @main def Test =
 

tests/run/i9530.scala

@@ -8,7 +8,7 @@ trait Scope:
 extension (using s: Scope)(expr: s.Expr)
    def show = expr.toString
    def eval = s.value(expr)
-   def *: (other: s.Expr) = s.combine(other, expr)
+   def *: (other: s.Expr) = s.combine(expr, other)
 
 def f(using s: Scope)(x: s.Expr): (String, s.Value) =
    (x.show, x.eval)

tests/run/instances.scala

@@ -31,7 +31,7 @@ object Test extends App {
   extension [T](xs: List[List[T]])
     def flattened = xs.foldLeft[List[T]](Nil)(_ ++ _)
 
-  extension [T](xs: Seq[T]) def :: (x: T) = x +: xs
+  extension [T](x: T) def :: (xs: Seq[T]) = x +: xs
 
   val ss: Seq[Int] = List(1, 2, 3)
   val ss1 = 0 :: ss