Fix toFunctionType to handle erased function types

Author: nicolasstucki

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

@@ -1475,6 +1475,7 @@ class Definitions {
   def PolyFunctionType = PolyFunctionClass.typeRef
 
   lazy val ErasedFunctionClass = requiredClass("scala.runtime.ErasedFunction")
+  def ErasedFunctionType = ErasedFunctionClass.typeRef
 
   /** If `cls` is a class in the scala package, its name, otherwise EmptyTypeName */
   def scalaClassName(cls: Symbol)(using Context): TypeName = cls.denot match

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

@@ -1875,7 +1875,7 @@ object Types {
      *  @param alwaysDependent if true, always create a dependent function type.
      */
     def toFunctionType(isJava: Boolean, dropLast: Int = 0, alwaysDependent: Boolean = false)(using Context): Type = this match {
-      case mt: MethodType if !mt.isParamDependent =>
+      case mt: MethodType if !mt.isParamDependent && !mt.hasErasedParams =>
         val formals1 = if (dropLast == 0) mt.paramInfos else mt.paramInfos dropRight dropLast
         val isContextual = mt.isContextualMethod && !ctx.erasedTypes
         val result1 = mt.nonDependentResultApprox match {
@@ -1888,6 +1888,9 @@ object Types {
         if alwaysDependent || mt.isResultDependent then
           RefinedType(funType, nme.apply, mt)
         else funType
+      case mt: MethodType if !mt.isParamDependent =>
+        assert(mt.hasErasedParams)
+        RefinedType(defn.ErasedFunctionType, nme.apply, mt)
       case poly @ PolyType(_, mt: MethodType) if !mt.isParamDependent =>
         RefinedType(defn.PolyFunctionType, nme.apply, poly)
     }