Avoid warnings about unused values

build.sbt

@@ -67,7 +67,9 @@ lazy val runtime = (projectMatrix in file("scalapb-runtime"))
       ProblemFilters.exclude[ReversedMissingMethodProblem]("scalapb.GeneratedEnum.asRecognized"),
       ProblemFilters.exclude[InheritedNewAbstractMethodProblem]("*Extension*"),
       ProblemFilters.exclude[Problem]("scalapb.options.*"),
-      ProblemFilters.exclude[FinalMethodProblem]("*.parseFrom")
+      ProblemFilters.exclude[FinalMethodProblem]("*.parseFrom"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem]("scalapb.UnknownFieldSet#Builder.parseField"),
+      ProblemFilters.exclude[IncompatibleResultTypeProblem]("scalapb.UnknownFieldSet#Field#Builder.parseField"),
     )
   )
   .jvmPlatform(
@@ -381,13 +383,14 @@ lazy val e2e = (projectMatrix in file("e2e"))
   )
   .settings(e2eCommonSettings)
   .settings(
-    scalacOptions ++= (if (!isScala3.value)
+    Compile / scalacOptions ++= (if (!isScala3.value)
                          Seq(
                            "-P:silencer:globalFilters=value deprecatedInt32 in class TestDeprecatedFields is deprecated",
                            "-P:silencer:pathFilters=custom_options_use;CustomAnnotationProto.scala;TestDeprecatedFields.scala",
                            "-P:silencer:lineContentFilters=import com.thesamet.pb.MisplacedMapper.weatherMapper"
-                         )
+                         ) ++ (if (scalaVersion.value.startsWith("2.13")) Seq("-Wnonunit-statement", "-Xlint") else Nil)
                        else Nil),
+    Test / scalacOptions ++= (if (scalaVersion.value.startsWith("2.13")) Seq("-Wnonunit-statement:false", "-Xlint:-multiarg-infix") else Nil),
     PB.protocVersion := versions.protobuf,
     Compile / PB.protocOptions += "--experimental_allow_proto3_optional",
     Compile / PB.targets := Seq(

compiler-plugin/src/main/scala/scalapb/compiler/ParseFromGenerator.scala

@@ -212,7 +212,9 @@ private[compiler] class ParseFromGenerator(
                        |    }""".stripMargin)
             } else p
           }
-          .when(!message.preservesUnknownFields)(_.add("    case tag => _input__.skipField(tag)"))
+          .when(!message.preservesUnknownFields)(
+            _.add("    case tag => _input__.skipField(tag): Unit")
+          )
           .when(message.preservesUnknownFields)(
             _.add(
               """    case tag =>

proptest/src/test/scala/GenTypes.scala

@@ -68,11 +68,13 @@ object GenTypes {
     def packable = false
     def isMap    = false
   }
+  object MessageReference extends (Int => ProtoType)
 
   case class EnumReference(id: Int) extends ProtoType {
     def packable = true
     def isMap    = false
   }
+  object EnumReference extends (Int => ProtoType)
 
   case class MapType(keyType: ProtoType, valueType: ProtoType) extends ProtoType {
     def packable = false

scalapb-runtime/src/main/scala/scalapb/UnknownFieldSet.scala

@@ -89,7 +89,7 @@ object UnknownFieldSet {
           lengthDelimited = lengthDelimited.result()
         )
 
-      def parseField(tag: Int, input: CodedInputStream) = {
+      def parseField(tag: Int, input: CodedInputStream): this.type = {
         val wireType = WireType.getTagWireType(tag)
 
         wireType match {
@@ -106,6 +106,7 @@ object UnknownFieldSet {
               s"Protocol message tag had invalid wire type: ${wireType}"
             )
         }
+        this
       }
     }
 
@@ -135,9 +136,10 @@ object UnknownFieldSet {
       if (fieldBuilders.isEmpty) UnknownFieldSet.empty
       else new UnknownFieldSet(fieldBuilders.view.mapValues(_.result()).toMap)
 
-    def parseField(tag: Int, input: CodedInputStream) = {
+    def parseField(tag: Int, input: CodedInputStream): this.type = {
       val fieldNumber = WireType.getTagFieldNumber(tag)
       fieldBuilders.getOrElseUpdate(fieldNumber, new Field.Builder()).parseField(tag, input)
+      this
     }
   }
 }