Fix desugaring of context bounds in extensions

Fixes scala#11586
Unfixes scala#11583

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

@@ -218,10 +218,27 @@ object desugar {
    *      def f$default$2[T](x: Int) = x + "m"
    */
   private def defDef(meth: DefDef, isPrimaryConstructor: Boolean = false)(using Context): Tree =
-    addDefaultGetters(elimContextBounds(meth, isPrimaryConstructor))
+    addDefaultGetters(elimContextBounds(Nil, meth, isPrimaryConstructor))
 
-  private def elimContextBounds(meth: DefDef, isPrimaryConstructor: Boolean)(using Context): DefDef =
+  private def defDef(extParamss: List[ParamClause], meth: DefDef)(using Context): Tree =
+    addDefaultGetters(elimContextBounds(extParamss, meth, false))
+
+  private def elimContextBounds(extParamss: List[ParamClause], meth: DefDef, isPrimaryConstructor: Boolean)(using Context): DefDef =
     val DefDef(_, paramss, tpt, rhs) = meth
+
+    rhs match
+      case MacroTree(call) =>
+        cpy.DefDef(meth)(rhs = call).withMods(meth.mods | Macro | Erased)
+      case _ =>
+        cpy.DefDef(meth)(
+          name = normalizeName(meth, tpt).asTermName,
+          paramss =
+            elimContextBounds(extParamss, isPrimaryConstructor) ++
+            elimContextBounds(paramss, isPrimaryConstructor)
+        )
+  end elimContextBounds
+
+  private def elimContextBounds(paramss: List[ParamClause], isPrimaryConstructor: Boolean)(using Context): List[ParamClause] =
     val evidenceParamBuf = ListBuffer[ValDef]()
 
     def desugarContextBounds(rhs: Tree): Tree = rhs match
@@ -240,15 +257,7 @@ object desugar {
         tparam => cpy.TypeDef(tparam)(rhs = desugarContextBounds(tparam.rhs))
       }(identity)
 
-    rhs match
-      case MacroTree(call) =>
-        cpy.DefDef(meth)(rhs = call).withMods(meth.mods | Macro | Erased)
-      case _ =>
-        addEvidenceParams(
-          cpy.DefDef(meth)(
-            name = normalizeName(meth, tpt).asTermName,
-            paramss = paramssNoContextBounds),
-          evidenceParamBuf.toList)
+    addEvidenceParams(paramssNoContextBounds, evidenceParamBuf.toList)
   end elimContextBounds
 
   def addDefaultGetters(meth: DefDef)(using Context): Tree =
@@ -348,22 +357,22 @@ object desugar {
     adaptToExpectedTpt(tree)
   }
 
-  /**  Add all evidence parameters in `params` as implicit parameters to `meth`.
-   *   If the parameters of `meth` end in an implicit parameter list or using clause,
+  /**  Add all evidence parameters in `params` as implicit parameters to `paramss`.
+   *   If the parameters of `paramss` end in an implicit parameter list or using clause,
    *   evidence parameters are added in front of that list. Otherwise they are added
    *   as a separate parameter clause.
    */
-  private def addEvidenceParams(meth: DefDef, params: List[ValDef])(using Context): DefDef =
-    params match
-      case Nil =>
-        meth
-      case evidenceParams =>
-        val paramss1 = meth.paramss.reverse match
-          case ValDefs(vparams @ (vparam :: _)) :: rparamss if vparam.mods.isOneOf(GivenOrImplicit) =>
-            ((evidenceParams ++ vparams) :: rparamss).reverse
-          case _ =>
-            meth.paramss :+ evidenceParams
-        cpy.DefDef(meth)(paramss = paramss1)
+
+  private def addEvidenceParams(paramss: List[ParamClause], params: List[ValDef])(using Context): List[ParamClause] =
+    paramss.reverse match
+      case ValDefs(vparams @ (vparam :: _)) :: rparamss if vparam.mods.isOneOf(GivenOrImplicit) =>
+        ((params ++ vparams) :: rparamss).reverse
+      case _ =>
+        params match
+          case Nil =>
+            paramss
+          case evidenceParams =>
+            paramss :+ evidenceParams
 
   /** The implicit evidence parameters of `meth`, as generated by `desugar.defDef` */
   private def evidenceParams(meth: DefDef)(using Context): List[ValDef] =
@@ -487,9 +496,8 @@ object desugar {
         case ddef: DefDef if ddef.name.isConstructorName =>
           decompose(
             defDef(
-              addEvidenceParams(
-                cpy.DefDef(ddef)(paramss = joinParams(constrTparams, ddef.paramss)),
-                evidenceParams(constr1).map(toDefParam(_, keepAnnotations = false, keepDefault = false)))))
+              cpy.DefDef(ddef)(paramss = addEvidenceParams(joinParams(constrTparams, ddef.paramss),
+              evidenceParams(constr1).map(toDefParam(_, keepAnnotations = false, keepDefault = false))))))
         case stat =>
           stat
       }
@@ -906,34 +914,29 @@ object desugar {
   /** Transform extension construct to list of extension methods */
   def extMethods(ext: ExtMethods)(using Context): Tree = flatTree {
     for mdef <- ext.methods yield
-      defDef(
-        cpy.DefDef(mdef)(
-          name = normalizeName(mdef, ext).asTermName,
-          paramss = mdef.paramss match
-            case params1 :: paramss1 if mdef.name.isRightAssocOperatorName =>
-              def badRightAssoc(problem: String) =
-                report.error(i"right-associative extension method $problem", mdef.srcPos)
-                ext.paramss ++ mdef.paramss
-              def noVParam = badRightAssoc("must start with a single parameter")
-              def checkVparam(params: ParamClause) = params match
-                case ValDefs(vparam :: Nil) =>
-                  if !vparam.mods.is(Given) then
-                    val (leadingUsing, otherExtParamss) = ext.paramss.span(isUsingOrTypeParamClause)
-                    leadingUsing ::: params1 :: otherExtParamss ::: paramss1
-                  else badRightAssoc("cannot start with using clause")
-                case _ =>
-                  noVParam
-              params1 match
-                case TypeDefs(_) => paramss1 match
-                  case params2 :: _ => checkVparam(params2)
-                  case _            => noVParam
-                case _ =>
-                  checkVparam(params1)
-
+      def ret(ess: List[ParamClause], mss: List[ParamClause]) =
+        defDef(
+          ess,
+          cpy.DefDef(mdef)(
+            name = normalizeName(mdef, ext).asTermName,
+            paramss = mss
+          ).withMods(mdef.mods | ExtensionMethod)
+        )
+      mdef.paramss match
+        case params1 :: paramss1 if mdef.name.isRightAssocOperatorName =>
+          def badRightAssoc(problem: String) =
+            report.error(i"right-associative extension method $problem", mdef.srcPos)
+            ret(ext.paramss, mdef.paramss)
+          params1 match
+            case ValDefs(vparam :: Nil) =>
+              if !vparam.mods.is(Given) then
+                val (leadingUsing, otherExtParamss) = ext.paramss.span(isUsingOrTypeParamClause)
+                ret(Nil, leadingUsing ::: params1 :: otherExtParamss ::: paramss1)
+              else badRightAssoc("cannot start with using clause")
             case _ =>
-              ext.paramss ++ mdef.paramss
-        ).withMods(mdef.mods | ExtensionMethod)
-      )
+              badRightAssoc("must start with a single parameter")
+        case _ =>
+          ret(ext.paramss, mdef.paramss)
   }
 
   /** Transforms

tests/pos/i11586.scala

@@ -0,0 +1,12 @@
+import language.future
+
+type Conv[T] = [X] =>> X => T
+
+trait SemiGroup[T]:
+  extension [U: Conv[T]](x: U)
+    def * (y: T): T
+  extension (x: T)
+    def * [U: Conv[T]](y: U): T
+
+trait Q[T, R: SemiGroup] extends SemiGroup[T]:
+  def res(x: R, y: R) = x * y