Add back type inference for erased parameters in closures

Also stops the type inference from automatically marking parameters as erased.

Author: natsukagami

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

@@ -1710,6 +1710,8 @@ object Types {
         else NoType
       case t if defn.isNonRefinedFunction(t) =>
         t
+      case t if defn.isErasedFunctionType(t) =>
+        t
       case t @ SAMType(_) =>
         t
       case _ =>

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1238,6 +1238,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         case RefinedType(parent, nme.apply, mt @ MethodTpe(_, formals, restpe))
         if defn.isNonRefinedFunction(parent) && formals.length == defaultArity =>
           (formals, untpd.DependentTypeTree(syms => restpe.substParams(mt, syms.map(_.termRef))))
+        case RefinedType(parent, nme.apply, mt @ MethodTpe(_, formals, restpe))
+        if defn.isErasedFunctionType(parent) && formals.length == defaultArity =>
+          (formals, untpd.DependentTypeTree(syms => restpe.substParams(mt, syms.map(_.termRef))))
         case SAMType(mt @ MethodTpe(_, formals, restpe)) =>
           (formals,
            if (mt.isResultDependent)
@@ -1509,8 +1512,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                   .withSpan(param.span.endPos)
               )
             val isErased = isProtoErased || paramType.hasAnnotation(defn.ErasedParamAnnot)
-            val param1 = if isErased then param.withAddedFlags(Flags.Erased) else param
-            cpy.ValDef(param1)(tpt = paramTpt)
+            cpy.ValDef(param)(tpt = paramTpt)
       desugared = desugar.makeClosure(inferredParams, fnBody, resultTpt, isContextual, tree.span)
 
     typed(desugared, pt)