Cleanup context bounds for poly functions implementation after review

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

@@ -350,11 +350,13 @@ object desugar {
         val Function(vparams: List[untpd.ValDef] @unchecked, rhs) = fun: @unchecked
         val newParamss = paramssNoContextBounds(tparams :: vparams :: Nil)
         val params = evidenceParamBuf.toList
-        if params.isEmpty then return meth
-        val boundNames = getBoundNames(params, newParamss)
-        val recur = fitEvidenceParams(params, nme.apply, boundNames)
-        val (paramsFst, paramsSnd) = recur(newParamss)
-        functionsOf((paramsFst ++ paramsSnd).filter(_.nonEmpty), rhs)
+        if params.isEmpty then
+          meth
+        else
+          val boundNames = getBoundNames(params, newParamss)
+          val recur = fitEvidenceParams(params, nme.apply, boundNames)
+          val (paramsFst, paramsSnd) = recur(newParamss)
+          functionsOf((paramsFst ++ paramsSnd).filter(_.nonEmpty), rhs)
   end elimContextBounds
 
   def addDefaultGetters(meth: DefDef)(using Context): Tree =
@@ -487,8 +489,27 @@ object desugar {
       case Ident(name: TermName) => names.contains(name)
       case _ => false
 
-  /** Fit evidence `params` into the `mparamss` parameter lists */
-  private def fitEvidenceParams(params: List[ValDef], methName: Name, boundNames: Set[TermName])(mparamss: List[ParamClause])(using Context): (List[ParamClause], List[ParamClause]) = mparamss match
+  /** Fit evidence `params` into the `mparamss` parameter lists, making sure
+   * that all parameters referencing `params` are after them.
+   * - for methods the final parameter lists are := result._1 ++ result._2
+   * - for poly functions, each element of the pair contains at most one term
+   *   parameter list
+   *
+   * @param params the evidence parameters list that should fit into `mparamss`
+   * @param methName the name of the method that `mparamss` belongs to
+   * @param boundNames the names of the evidence parameters
+   * @param mparamss the original parameter lists of the method
+   * @return a pair of parameter lists containing all parameter lists in a
+   * reference-correct order; make sure that `params` is always at the
+   * intersection of the pair elements; this is relevant, for poly functions
+   * where `mparamss` is guaranteed to have exectly one term parameter list,
+   * then each pair element will have at most one term parameter list
+   */
+  private def fitEvidenceParams(
+    params: List[ValDef],
+    methName: Name,
+    boundNames: Set[TermName]
+  )(mparamss: List[ParamClause])(using Context): (List[ParamClause], List[ParamClause]) = mparamss match
     case ValDefs(mparams) :: _ if mparams.exists(referencesName(_, boundNames)) =>
       (params :: Nil) -> mparamss
     case ValDefs(mparams @ (mparam :: _)) :: Nil if mparam.mods.isOneOf(GivenOrImplicit) =>
@@ -547,19 +568,20 @@ object desugar {
 
     val boundNames = getBoundNames(params, meth.paramss)
 
-    val recur = fitEvidenceParams(params, meth.name, boundNames)
+    val fitParams = fitEvidenceParams(params, meth.name, boundNames)
 
-    if meth.hasAttachment(PolyFunctionApply) then
-      meth.removeAttachment(PolyFunctionApply)
-      // for PolyFunctions we are limited to a single term param list, so we reuse the recur logic to compute the new parameter lists
-      // and then we add the other parameter lists as function types to the return type
-      val (paramsFst, paramsSnd) = recur(meth.paramss)
+    if meth.removeAttachment(PolyFunctionApply).isDefined then
+      // for PolyFunctions we are limited to a single term param list, so we
+      // reuse the fitEvidenceParams logic to compute the new parameter lists
+      // and then we add the other parameter lists as function types to the
+      // return type
+      val (paramsFst, paramsSnd) = fitParams(meth.paramss)
       if ctx.mode.is(Mode.Type) then
         cpy.DefDef(meth)(paramss = paramsFst, tpt = functionsOf(paramsSnd, meth.tpt))
       else
         cpy.DefDef(meth)(paramss = paramsFst, rhs = functionsOf(paramsSnd, meth.rhs))
     else
-      val (paramsFst, paramsSnd) = recur(meth.paramss)
+      val (paramsFst, paramsSnd) = fitParams(meth.paramss)
       cpy.DefDef(meth)(paramss = paramsFst ++ paramsSnd)
   end addEvidenceParams