Correct handling of overloaded methods with PolyFunction args

Overloaded methods must have distinct signatures after erasure. The
check for this happens before PolyFunctions have been mapped to the
corresponding FunctionN so will always be considered as erasing to the
same type (PolyFunction without any refinement).

To avoid this we perform the PolyFunction to FunctionN mapping in the
method signature computation.

Author: milessabin

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3194,8 +3194,17 @@ object Types {
       companion.eq(ContextualMethodType) ||
       companion.eq(ErasedContextualMethodType)
 
+
     def computeSignature(implicit ctx: Context): Signature = {
-      val params = if (isErasedMethod) Nil else paramInfos
+      def polyFunctionSignature(tp: Type): Type = tp match {
+        case RefinedType(parent, nme.apply, refinedInfo) if parent.typeSymbol eq defn.PolyFunctionClass =>
+          val res = refinedInfo.resultType
+          val paramss = res.paramNamess
+          defn.FunctionType(paramss.head.length)
+        case _ => tp
+      }
+
+      val params = if (isErasedMethod) Nil else paramInfos.mapConserve(polyFunctionSignature)
       resultSignature.prepend(params, isJavaMethod)
     }