Fix how unpickler reads an erased function type

Author: natsukagami

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -320,7 +320,7 @@ class TreeUnpickler(reader: TastyReader,
       }
 
       def readLengthType(): Type = {
-        val end = readEnd()
+        var end = readEnd()
 
         def readMethodic[N <: Name, PInfo <: Type, LT <: LambdaType]
             (companionOp: FlagSet => LambdaTypeCompanion[N, PInfo, LT], nameMap: Name => N): LT = {
@@ -400,11 +400,11 @@ class TreeUnpickler(reader: TastyReader,
             case POLYtype =>
               readMethodic(_ => PolyType, _.toTypeName)
             case ERASEDMETHODtype =>
-              val isErased =
-                val end = readEnd()
-                collectWhile(end != currentAddr) {
-                  readByte() == 1
-                }
+              // The first `end` was the length of the erased parameter list, read and fix it.
+              val isErased = collectWhile(end != currentAddr) {
+                readByte() == 1
+              }
+              end = readEnd()
               def methodTypeCompanion(mods: FlagSet): MethodTypeCompanion =
                 if mods.is(Implicit) then ErasedImplicitMethodType(isErased)
                 else if mods.is(Given) then ErasedContextualMethodType(isErased)