Fix handling of right associative extension methods across scaladoc and printers

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

@@ -988,7 +988,9 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
                 //   - 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 (leftTyParams, rest1) = tree.paramss match
+                  case fst :: tail if isTypeParamClause(fst) => (List(fst), tail)
+                  case other => (List(), other)
                 val (leadingUsing, rest2) = rest1.span(isUsingClause)
                 val (rightTyParams, rest3) = rest2.span(isTypeParamClause)
                 val (rightParam, rest4) = rest3.splitAt(1)

presentation-compiler/src/main/dotty/tools/pc/printer/ShortenedTypePrinter.scala

@@ -419,7 +419,9 @@ class ShortenedTypePrinter(
     if gsym.is(Flags.ExtensionMethod) then
       val filteredParams =
         if gsym.name.isRightAssocOperatorName then
-          val (leadingTyParamss, rest1) = paramss.span(isTypeParamClause)
+          val (leadingTyParamss, rest1) = paramss match
+            case fst :: tail if isTypeParamClause(fst) => (List(fst), tail)
+            case other => (List(), other)
           val (leadingUsing, rest2) = rest1.span(isUsingClause)
           val (rightTyParamss, rest3) = rest2.span(isTypeParamClause)
           val (rightParamss, rest4) = rest3.splitAt(1)

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

@@ -683,3 +683,14 @@ class HoverTermSuite extends BaseHoverSuite:
         |""".stripMargin,
       """yy: A{type T = Int}""".stripMargin.hover
   )
+
+  @Test def `right-assoc-extension`: Unit =
+    check(
+      """
+        |case class Wrap[+T](x: T)
+        |
+        |extension [T](a: T)
+        |  def <<*@@:>>[U <: Tuple](b: Wrap[U]): Wrap[T *: U] = Wrap(a *: b.x)
+        |""".stripMargin,
+      "extension [T](a: T) def *:[U <: Tuple](b: Wrap[U]): Wrap[T *: U]".hover
+    )

scaladoc-testcases/src/tests/rightAssocExtension.scala

@@ -0,0 +1,7 @@
+package tests.rightAssocExtension
+
+case class Wrap[+T](x: T)
+
+extension [T](a: T)
+  def *:[U <: Tuple](b: Wrap[U]): Wrap[T *: U]
+    = Wrap(a *: b.x)

scaladoc/src/dotty/tools/scaladoc/tasty/ClassLikeSupport.scala

@@ -359,7 +359,9 @@ trait ClassLikeSupport:
       if methodSymbol.isExtensionMethod && methodSymbol.isRightAssoc then
         // Taken from RefinedPrinter.scala
         // If you change the names of the clauses below, also change them in right-associative-extension-methods.md
-        val (leftTyParams, rest1) = memberInfo.paramLists.span(_.isType)
+        val (leftTyParams, rest1) = memberInfo.paramLists match
+          case fst :: tail if fst.isType => (List(fst), tail)
+          case other => (List(), other)
         val (leadingUsing, rest2) = rest1.span(_.isUsing)
         val (rightTyParams, rest3) = rest2.span(_.isType)
         val (rightParam, rest4) = rest3.splitAt(1)

scaladoc/test/dotty/tools/scaladoc/signatures/TranslatableSignaturesTestCases.scala

@@ -122,3 +122,5 @@ class InfixTypes extends SignatureTest("infixTypes", SignatureTest.all)
 class ExtendsCall extends SignatureTest("extendsCall", SignatureTest.all)
 
 class RefinedFunctionTypes extends SignatureTest("refinedFunctionTypes", SignatureTest.all)
+
+class RightAssocExtension extends SignatureTest("rightAssocExtension", SignatureTest.all)