Make right assoc ext method fwd refs error

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

@@ -1099,8 +1099,8 @@ object desugar {
           paramss match
             case rightParam :: paramss1 => // `rightParam` must have a single parameter and without `given` flag
 
-              def badRightAssoc(problem: String) =
-                report.error(em"right-associative extension method $problem", mdef.srcPos)
+              def badRightAssoc(problem: String, pos: SrcPos) =
+                report.error(em"right-associative extension method $problem", pos)
                 extParamss ++ mdef.paramss
 
               rightParam match
@@ -1116,11 +1116,23 @@ object desugar {
                     //
                     // If you change the names of the clauses below, also change them in right-associative-extension-methods.md
                     val (leftTyParamsAndLeadingUsing, leftParamAndTrailingUsing) = extParamss.span(isUsingOrTypeParamClause)
+
+                    val names = (for ps <- mdef.paramss; p <- ps yield p.name).toSet[Name]
+
+                    val tt = new untpd.UntypedTreeTraverser:
+                      def traverse(tree: Tree)(using Context): Unit = tree match
+                        case tree: Ident if names.contains(tree.name) =>
+                          badRightAssoc(s"cannot have a forward reference to ${tree.name}", tree.srcPos)
+                        case _ => traverseChildren(tree)
+
+                    for ts <- leftParamAndTrailingUsing; t <- ts do
+                      tt.traverse(t)
+
                     leftTyParamsAndLeadingUsing ::: rightTyParams ::: rightParam :: leftParamAndTrailingUsing ::: paramss1
                   else
-                    badRightAssoc("cannot start with using clause")
+                    badRightAssoc("cannot start with using clause", mdef.srcPos)
                 case _ =>
-                  badRightAssoc("must start with a single parameter")
+                  badRightAssoc("must start with a single parameter", mdef.srcPos)
             case _ =>
               // no value parameters, so not an infix operator.
               extParamss ++ mdef.paramss

tests/neg/i16815.check

@@ -0,0 +1,28 @@
+-- Error: tests/neg/i16815.scala:3:37 ----------------------------------------------------------------------------------
+3 |extension [C1 >: Chain <: Chain](c2: c1.Tail) // error
+  |                                     ^^
+  |                                     right-associative extension method cannot have a forward reference to c1
+-- Error: tests/neg/i16815.scala:6:24 ----------------------------------------------------------------------------------
+6 |extension [C1](c2: (C1, C2)) // error
+  |                        ^^
+  |                        right-associative extension method cannot have a forward reference to C2
+-- Error: tests/neg/i16815.scala:9:19 ----------------------------------------------------------------------------------
+9 |extension [C1](c2: C2) // error
+  |                   ^^
+  |                   right-associative extension method cannot have a forward reference to C2
+-- Error: tests/neg/i16815.scala:12:24 ---------------------------------------------------------------------------------
+12 |extension [C1](c2: (C1, C2, C3)) // error // error
+   |                        ^^
+   |                        right-associative extension method cannot have a forward reference to C2
+-- Error: tests/neg/i16815.scala:12:28 ---------------------------------------------------------------------------------
+12 |extension [C1](c2: (C1, C2, C3)) // error // error
+   |                            ^^
+   |                            right-associative extension method cannot have a forward reference to C3
+-- Error: tests/neg/i16815.scala:15:48 ---------------------------------------------------------------------------------
+15 |extension [C1](str: String)(using z: (str.type, C2)) // error
+   |                                                ^^
+   |                                        right-associative extension method cannot have a forward reference to C2
+-- Error: tests/neg/i16815.scala:19:31 ---------------------------------------------------------------------------------
+19 |extension [D1 <: Int](D2: (D1, D2)) // error
+   |                               ^^
+   |                               right-associative extension method cannot have a forward reference to D2

tests/neg/i16815.scala

@@ -0,0 +1,20 @@
+trait Chain { type Tail <: Chain }
+
+extension [C1 >: Chain <: Chain](c2: c1.Tail) // error
+  def ra1_:[C2 <: C1](c1: C1): C2 = ???
+
+extension [C1](c2: (C1, C2)) // error
+  def ra2_:[C2 <: C1](c1: (C1, C2)): C2 = ???
+
+extension [C1](c2: C2) // error
+  def ra3_:[C2 <: C1](c1: C1): C2 = ???
+
+extension [C1](c2: (C1, C2, C3)) // error // error
+  def ra4_:[C2 <: C1, C3 <: C1](c1: (C1, C2)): C2 = ???
+
+extension [C1](str: String)(using z: (str.type, C2)) // error
+  def ra5_:[C2 <: Int](c1: C1): C2 = ???
+
+type D2 = String
+extension [D1 <: Int](D2: (D1, D2)) // error
+  def sa2_:[D2 <: D1](D1: (D1, D2)): D2 = ???