Infer erasedness for desugared closure even if the type is known

Author: natsukagami

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

@@ -1500,21 +1500,22 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     if desugared.isEmpty then
       val inferredParams: List[untpd.ValDef] =
         for ((param, i) <- params.zipWithIndex) yield
-          if (!param.tpt.isEmpty) param
-          else
-            val (formal, isErased) = protoFormal(i)
-            val knownFormal = isFullyDefined(formal, ForceDegree.failBottom)
-            val paramType =
-              if knownFormal then formal
-              else inferredFromTarget(param, formal, calleeType, paramIndex)
-                .orElse(errorType(AnonymousFunctionMissingParamType(param, tree, formal), param.srcPos))
-            val paramTpt = untpd.TypedSplice(
-                (if knownFormal then InferredTypeTree() else untpd.TypeTree())
-                  .withType(paramType.translateFromRepeated(toArray = false))
-                  .withSpan(param.span.endPos)
-              )
-            val param1 = if isErased then param.withAddedFlags(Flags.Erased) else param
-            cpy.ValDef(param1)(tpt = paramTpt)
+          val (formal, isErased) = protoFormal(i)
+          val param0 =
+            if (!param.tpt.isEmpty) param
+            else
+              val knownFormal = isFullyDefined(formal, ForceDegree.failBottom)
+              val paramType =
+                if knownFormal then formal
+                else inferredFromTarget(param, formal, calleeType, paramIndex)
+                  .orElse(errorType(AnonymousFunctionMissingParamType(param, tree, formal), param.srcPos))
+              val paramTpt = untpd.TypedSplice(
+                  (if knownFormal then InferredTypeTree() else untpd.TypeTree())
+                    .withType(paramType.translateFromRepeated(toArray = false))
+                    .withSpan(param.span.endPos)
+                )
+              cpy.ValDef(param)(tpt = paramTpt)
+          if isErased then param0.withAddedFlags(Flags.Erased) else param0
       desugared = desugar.makeClosure(inferredParams, fnBody, resultTpt, isContextual, tree.span)
 
     typed(desugared, pt)