Try to figure out erasedness from the parameter's types as well

Author: natsukagami

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

@@ -1506,7 +1506,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         for ((param, i) <- params.zipWithIndex) yield
           if (!param.tpt.isEmpty) param
           else
-            val (formal, isErased) = protoFormal(i)
+            val (formal, isProtoErased) = protoFormal(i)
             val knownFormal = isFullyDefined(formal, ForceDegree.failBottom)
             val paramType =
               if knownFormal then formal
@@ -1517,6 +1517,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                   .withType(paramType.translateFromRepeated(toArray = false))
                   .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)
       desugared = desugar.makeClosure(inferredParams, fnBody, resultTpt, isContextual, tree.span)