diff --git a/compiler/src/dotty/tools/MainGenericCompiler.scala b/compiler/src/dotty/tools/MainGenericCompiler.scala
index 2c3f6f97e79e..98bd9078c397 100644
--- a/compiler/src/dotty/tools/MainGenericCompiler.scala
+++ b/compiler/src/dotty/tools/MainGenericCompiler.scala
@@ -85,7 +85,7 @@ case class CompileSettings(
 
 object MainGenericCompiler {
 
-  val classpathSeparator = File.pathSeparator
+  val classpathSeparator: String = File.pathSeparator
 
   @sharable val javaOption = raw"""-J(.*)""".r
   @sharable val javaPropOption = raw"""-D(.+?)=(.?)""".r
diff --git a/compiler/src/dotty/tools/MainGenericRunner.scala b/compiler/src/dotty/tools/MainGenericRunner.scala
index b32630a5d63b..9ff96f812bca 100644
--- a/compiler/src/dotty/tools/MainGenericRunner.scala
+++ b/compiler/src/dotty/tools/MainGenericRunner.scala
@@ -96,7 +96,7 @@ case class Settings(
 
 object MainGenericRunner {
 
-  val classpathSeparator = File.pathSeparator
+  val classpathSeparator: String = File.pathSeparator
 
   def processClasspath(cp: String, tail: List[String]): (List[String], List[String]) =
     val cpEntries = cp.split(classpathSeparator).toList
diff --git a/compiler/src/dotty/tools/backend/jvm/CodeGen.scala b/compiler/src/dotty/tools/backend/jvm/CodeGen.scala
index 1e2abbcff866..093d4f997aa7 100644
--- a/compiler/src/dotty/tools/backend/jvm/CodeGen.scala
+++ b/compiler/src/dotty/tools/backend/jvm/CodeGen.scala
@@ -152,7 +152,7 @@ class CodeGen(val int: DottyBackendInterface, val primitives: DottyPrimitives)(
     new interfaces.AbstractFile {
       override def name = absfile.name
       override def path = absfile.path
-      override def jfile = Optional.ofNullable(absfile.file)
+      override def jfile: Optional[java.io.File] = Optional.ofNullable(absfile.file)
     }
 
   private def genClass(cd: TypeDef, unit: CompilationUnit): ClassNode = {
diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index d534db7df1ce..004fc39e1298 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -2959,7 +2959,7 @@ object SymDenotations {
       dependent = null
     }
 
-    protected def addDependent(dep: InheritedCache) = {
+    protected def addDependent(dep: InheritedCache): Unit = {
       if (dependent == null) dependent = new WeakHashMap
       dependent.nn.put(dep, ())
     }
diff --git a/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala b/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
index 673874ae2769..40c6667fdd24 100644
--- a/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
+++ b/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
@@ -234,7 +234,7 @@ class InteractiveDriver(val settings: List[String]) extends Driver {
   private def classesFromDir(dir: Path, buffer: mutable.ListBuffer[TypeName]): Unit =
     try
       Files.walkFileTree(dir, new SimpleFileVisitor[Path] {
-        override def visitFile(path: Path, attrs: BasicFileAttributes) = {
+        override def visitFile(path: Path, attrs: BasicFileAttributes): FileVisitResult = {
           if (!attrs.isDirectory) {
             val name = path.getFileName.toString
             if name.endsWith(tastySuffix) then
diff --git a/compiler/src/dotty/tools/dotc/sbt/APIUtils.scala b/compiler/src/dotty/tools/dotc/sbt/APIUtils.scala
index 4ee1fd0f6b68..42843a89c0ab 100644
--- a/compiler/src/dotty/tools/dotc/sbt/APIUtils.scala
+++ b/compiler/src/dotty/tools/dotc/sbt/APIUtils.scala
@@ -17,10 +17,10 @@ import xsbti.api.SafeLazy.strict
  */
 object APIUtils {
   private object Constants {
-    val PublicAccess = api.Public.create()
-    val EmptyModifiers = new api.Modifiers(false, false, false, false, false, false, false, false)
-    val EmptyStructure = api.Structure.of(strict(Array.empty), strict(Array.empty), strict(Array.empty))
-    val EmptyType = api.EmptyType.of()
+    val PublicAccess: api.Public = api.Public.create()
+    val EmptyModifiers: api.Modifiers = new api.Modifiers(false, false, false, false, false, false, false, false)
+    val EmptyStructure: api.Structure = api.Structure.of(strict(Array.empty), strict(Array.empty), strict(Array.empty))
+    val EmptyType: api.EmptyType = api.EmptyType.of()
   }
 
   import Constants.*
diff --git a/compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala b/compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala
index 4d915b57df1b..5f3eeff91ff3 100644
--- a/compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala
+++ b/compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala
@@ -232,13 +232,13 @@ private class ExtractAPICollector(nonLocalClassSymbols: mutable.HashSet[Symbol])
 
   private object Constants {
     val emptyStringArray = Array[String]()
-    val local            = api.ThisQualifier.create()
-    val public           = api.Public.create()
-    val privateLocal     = api.Private.create(local)
-    val protectedLocal   = api.Protected.create(local)
-    val unqualified      = api.Unqualified.create()
-    val thisPath         = api.This.create()
-    val emptyType        = api.EmptyType.create()
+    val local: api.ThisQualifier = api.ThisQualifier.create()
+    val public: api.Public = api.Public.create()
+    val privateLocal: api.Private = api.Private.create(local)
+    val protectedLocal: api.Protected = api.Protected.create(local)
+    val unqualified: api.Unqualified = api.Unqualified.create()
+    val thisPath: api.This = api.This.create()
+    val emptyType: api.EmptyType = api.EmptyType.create()
     val emptyModifiers   =
       new api.Modifiers(false, false, false, false, false,false, false, false)
   }
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index 2d178e2f3773..ba472512a953 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -1194,6 +1194,18 @@ object RefChecks {
           report.warning(ExtensionNullifiedByMember(sym, target), sym.srcPos)
   end checkExtensionMethods
 
+  /** Check that public (and protected) methods/fields do not expose flexible types. */
+  def checkPublicFlexibleTypes(sym: Symbol)(using Context): Unit =
+    if ctx.explicitNulls && !ctx.isJava
+        && sym.exists && !sym.is(Private) && sym.owner.isClass
+        && !sym.isOneOf(Synthetic | InlineProxy | Param | Exported) then
+      val resTp = sym.info.finalResultType
+      if resTp.existsPart(_.isInstanceOf[FlexibleType], StopAt.Static) then
+        report.warning(
+          em"${sym.show} exposes a flexible type in its inferred result type ${resTp}. Consider annotating the type explicitly",
+          sym.srcPos
+        )
+
   /** Verify that references in the user-defined `@implicitNotFound` message are valid.
    *  (i.e. they refer to a type variable that really occurs in the signature of the annotated symbol.)
    */
@@ -1330,6 +1342,7 @@ class RefChecks extends MiniPhase { thisPhase =>
       val sym = tree.symbol
       checkNoPrivateOverrides(sym)
       checkVolatile(sym)
+      checkPublicFlexibleTypes(sym)
       if (sym.exists && sym.owner.isTerm) {
         tree.rhs match {
           case Ident(nme.WILDCARD) => report.error(UnboundPlaceholderParameter(), sym.srcPos)
@@ -1345,6 +1358,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     checkImplicitNotFoundAnnotation.defDef(sym.denot)
     checkUnaryMethods(sym)
     checkExtensionMethods(sym)
+    checkPublicFlexibleTypes(sym)
     tree
   }
 
diff --git a/compiler/src/dotty/tools/io/Path.scala b/compiler/src/dotty/tools/io/Path.scala
index 39665395c289..64137d691898 100644
--- a/compiler/src/dotty/tools/io/Path.scala
+++ b/compiler/src/dotty/tools/io/Path.scala
@@ -245,12 +245,12 @@ class Path private[io] (val jpath: JPath) {
     if (!exists) false
     else {
       Files.walkFileTree(jpath, new SimpleFileVisitor[JPath]() {
-        override def visitFile(file: JPath, attrs: BasicFileAttributes) = {
+        override def visitFile(file: JPath, attrs: BasicFileAttributes): FileVisitResult = {
           Files.delete(file)
           FileVisitResult.CONTINUE
         }
 
-        override def postVisitDirectory(dir: JPath, exc: IOException) = {
+        override def postVisitDirectory(dir: JPath, exc: IOException): FileVisitResult = {
           Files.delete(dir)
           FileVisitResult.CONTINUE
         }
diff --git a/compiler/src/dotty/tools/repl/AbstractFileClassLoader.scala b/compiler/src/dotty/tools/repl/AbstractFileClassLoader.scala
index 7a457a1d7546..1796a7dc68b5 100644
--- a/compiler/src/dotty/tools/repl/AbstractFileClassLoader.scala
+++ b/compiler/src/dotty/tools/repl/AbstractFileClassLoader.scala
@@ -26,7 +26,7 @@ class AbstractFileClassLoader(val root: AbstractFile, parent: ClassLoader) exten
   // on JDK 20 the URL constructor we're using is deprecated,
   // but the recommended replacement, URL.of, doesn't exist on JDK 8
   @annotation.nowarn("cat=deprecation")
-  override protected def findResource(name: String) =
+  override protected def findResource(name: String): URL | Null =
     findAbstractFile(name) match
       case null => null
       case file => new URL(null, s"memory:${file.path}", new URLStreamHandler {
@@ -35,13 +35,13 @@ class AbstractFileClassLoader(val root: AbstractFile, parent: ClassLoader) exten
           override def getInputStream = file.input
         }
       })
-  override protected def findResources(name: String) =
+  override protected def findResources(name: String): java.util.Enumeration[URL] =
     findResource(name) match
       case null => Collections.enumeration(Collections.emptyList[URL])  //Collections.emptyEnumeration[URL]
       case url  => Collections.enumeration(Collections.singleton(url))
 
   override def findClass(name: String): Class[?] = {
-    var file: AbstractFile = root
+    var file: AbstractFile | Null = root
     val pathParts = name.split("[./]").toList
     for (dirPart <- pathParts.init) {
       file = file.lookupName(dirPart, true)
diff --git a/compiler/src/dotty/tools/repl/JLineTerminal.scala b/compiler/src/dotty/tools/repl/JLineTerminal.scala
index e4ac1626525e..2680704d326e 100644
--- a/compiler/src/dotty/tools/repl/JLineTerminal.scala
+++ b/compiler/src/dotty/tools/repl/JLineTerminal.scala
@@ -106,7 +106,7 @@ class JLineTerminal extends java.io.Closeable {
     ) extends reader.ParsedLine {
       // Using dummy values, not sure what they are used for
       def wordIndex = -1
-      def words = java.util.Collections.emptyList[String]
+      def words: java.util.List[String] = java.util.Collections.emptyList[String]
     }
 
     def parse(input: String, cursor: Int, context: ParseContext): reader.ParsedLine = {
diff --git a/compiler/src/dotty/tools/repl/ScriptEngine.scala b/compiler/src/dotty/tools/repl/ScriptEngine.scala
index 7d385daa43e4..cce16000577f 100644
--- a/compiler/src/dotty/tools/repl/ScriptEngine.scala
+++ b/compiler/src/dotty/tools/repl/ScriptEngine.scala
@@ -64,20 +64,21 @@ class ScriptEngine extends AbstractScriptEngine {
 
 object ScriptEngine {
   import java.util.Arrays
+  import java.util.List
   import scala.util.Properties
 
   class Factory extends ScriptEngineFactory {
     def getEngineName = "Scala REPL"
     def getEngineVersion = "3.0"
-    def getExtensions = Arrays.asList("scala")
+    def getExtensions: List[String] = Arrays.asList("scala")
     def getLanguageName = "Scala"
     def getLanguageVersion = Properties.versionString
-    def getMimeTypes = Arrays.asList("application/x-scala")
-    def getNames = Arrays.asList("scala")
+    def getMimeTypes: List[String] = Arrays.asList("application/x-scala")
+    def getNames: List[String] = Arrays.asList("scala")
 
-    def getMethodCallSyntax(obj: String, m: String, args: String*) = s"$obj.$m(${args.mkString(", ")})"
+    def getMethodCallSyntax(obj: String, m: String, args: String*): String = s"$obj.$m(${args.mkString(", ")})"
 
-    def getOutputStatement(toDisplay: String) = s"""print("$toDisplay")"""
+    def getOutputStatement(toDisplay: String): String = s"""print("$toDisplay")"""
 
     def getParameter(key: String): Object = key match {
       case JScriptEngine.ENGINE           => getEngineName
@@ -88,7 +89,7 @@ object ScriptEngine {
       case _ => null
     }
 
-    def getProgram(statements: String*) = statements.mkString("; ")
+    def getProgram(statements: String*): String = statements.mkString("; ")
 
     def getScriptEngine: JScriptEngine = new ScriptEngine
   }
diff --git a/compiler/src/dotty/tools/tasty/besteffort/BestEffortTastyFormat.scala b/compiler/src/dotty/tools/tasty/besteffort/BestEffortTastyFormat.scala
index 99a24ce5f346..41876016b4ec 100644
--- a/compiler/src/dotty/tools/tasty/besteffort/BestEffortTastyFormat.scala
+++ b/compiler/src/dotty/tools/tasty/besteffort/BestEffortTastyFormat.scala
@@ -38,7 +38,7 @@ object BestEffortTastyFormat {
   // added AST tag - Best Effort TASTy only
   final val ERRORtype = 50
 
-  def astTagToString(tag: Int) = tag match {
+  def astTagToString(tag: Int): String = tag match {
     case ERRORtype => "ERRORtype"
     case _ => TastyFormat.astTagToString(tag)
   }
diff --git a/compiler/test/dotty/tools/dotc/TupleShowTests.scala b/compiler/test/dotty/tools/dotc/TupleShowTests.scala
index 88e0587d7d71..3aa9d0d84b42 100644
--- a/compiler/test/dotty/tools/dotc/TupleShowTests.scala
+++ b/compiler/test/dotty/tools/dotc/TupleShowTests.scala
@@ -54,24 +54,28 @@ class TupleShowTests extends DottyTest:
 
   @Test def tup3_show10 = chkEq("(Int,\n  Long,\n  Short)".normEOL, tup3.toText(ctx.printer).mkString(10, false))
 
-  val res21 = """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
-                 |  Int, Long, Long, Long, Long, Long, Int)""".stripMargin.normEOL
+  val res21: String =
+    """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
+       |  Int, Long, Long, Long, Long, Long, Int)""".stripMargin.normEOL
 
-  val res22 = """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
-                 |  Int, Long, Long, Long, Long, Long, Int, Long)""".stripMargin.normEOL
+  val res22: String =
+    """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
+       |  Int, Long, Long, Long, Long, Long, Int, Long)""".stripMargin.normEOL
 
-  val res23 = """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
-                 |  Int, Long, Long, Long, Long, Long, Int, Long, Short)""".stripMargin.normEOL
+  val res23: String =
+    """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
+       |  Int, Long, Long, Long, Long, Long, Int, Long, Short)""".stripMargin.normEOL
 
-  val res24 = """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
-                 |  Int, Long, Long, Long, Long, Long, Int, Long, Short, Short)""".stripMargin.normEOL
+  val res24: String =
+    """|(Int, Int, Int, Int, Int, Long, Long, Long, Long, Long, Int, Int, Int, Int,
+       |  Int, Long, Long, Long, Long, Long, Int, Long, Short, Short)""".stripMargin.normEOL
 
   def chkEq[A](expected: A, obtained: A) = assert(expected == obtained, diff(s"$expected", s"$obtained"))
 
   /** On Windows the string literal in this test source file will be read with `\n` (b/c of "-encoding UTF8")
   *   but the compiler will correctly emit \r\n as the line separator.
   *   So we align the expected result to faithfully compare test results. */
-  extension (str: String) def normEOL = if EOL == "\n" then str else str.replace("\n", EOL)
+  extension (str: String) def normEOL: String = if EOL == "\n" then str else str.replace("\n", EOL)
 
   def diff(exp: String, obt: String) =
     val min = math.min(exp.length, obt.length)
diff --git a/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala b/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
index 827a997af14b..6df830905603 100644
--- a/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
+++ b/compiler/test/dotty/tools/dotc/semanticdb/SemanticdbTests.scala
@@ -33,7 +33,7 @@ import dotty.tools.dotc.util.SourceFile
  *  only 1 semanticdb file should be present
  *  @param source the single source file producing the semanticdb
  */
-@main def metac(root: String, source: String) =
+@main def metac(root: String, source: String): Unit =
   val rootSrc = Paths.get(root)
   val sourceSrc = Paths.get(source)
   val semanticFile = FileSystems.getDefault.getPathMatcher("glob:**.semanticdb")
@@ -53,13 +53,13 @@ import dotty.tools.dotc.util.SourceFile
 
 @Category(Array(classOf[BootstrappedOnlyTests]))
 class SemanticdbTests:
-  val javaFile = FileSystems.getDefault.getPathMatcher("glob:**.java")
-  val scalaFile = FileSystems.getDefault.getPathMatcher("glob:**.scala")
-  val expectFile = FileSystems.getDefault.getPathMatcher("glob:**.expect.scala")
-  val rootSrc = Paths.get(System.getProperty("dotty.tools.dotc.semanticdb.test"))
-  val expectSrc = rootSrc.resolve("expect")
-  val javaRoot = rootSrc.resolve("javacp")
-  val metacExpectFile = rootSrc.resolve("metac.expect")
+  val javaFile: PathMatcher = FileSystems.getDefault.getPathMatcher("glob:**.java")
+  val scalaFile: PathMatcher = FileSystems.getDefault.getPathMatcher("glob:**.scala")
+  val expectFile: PathMatcher = FileSystems.getDefault.getPathMatcher("glob:**.expect.scala")
+  val rootSrc: Path = Paths.get(System.getProperty("dotty.tools.dotc.semanticdb.test"))
+  val expectSrc: Path = rootSrc.resolve("expect")
+  val javaRoot: Path = rootSrc.resolve("javacp")
+  val metacExpectFile: Path = rootSrc.resolve("metac.expect")
 
   @Category(Array(classOf[dotty.SlowTests]))
   @Test def expectTests: Unit = if (!scala.util.Properties.isWin) runExpectTest(updateExpectFiles = false)
diff --git a/compiler/test/dotty/tools/repl/AbstractFileClassLoaderTest.scala b/compiler/test/dotty/tools/repl/AbstractFileClassLoaderTest.scala
index 27796feb819b..44540629016e 100644
--- a/compiler/test/dotty/tools/repl/AbstractFileClassLoaderTest.scala
+++ b/compiler/test/dotty/tools/repl/AbstractFileClassLoaderTest.scala
@@ -29,7 +29,7 @@ class AbstractFileClassLoaderTest:
   // cf ScalaClassLoader#classBytes
   extension (loader: ClassLoader)
     // An InputStream representing the given class name, or null if not found.
-    def classAsStream(className: String) = loader.getResourceAsStream {
+    def classAsStream(className: String): InputStream = loader.getResourceAsStream {
       if className.endsWith(".class") then className
       else s"${className.replace('.', '/')}.class"  // classNameToPath
     }
diff --git a/compiler/test/dotty/tools/scripting/ScriptTestEnv.scala b/compiler/test/dotty/tools/scripting/ScriptTestEnv.scala
index 771c3ba14af0..387e309f6aec 100644
--- a/compiler/test/dotty/tools/scripting/ScriptTestEnv.scala
+++ b/compiler/test/dotty/tools/scripting/ScriptTestEnv.scala
@@ -282,7 +282,7 @@ object ScriptTestEnv {
   }
 
   extension(f: File) {
-    def name = f.getName
+    def name: String = f.getName
     def norm: String = f.toPath.normalize.norm
     def absPath: String = f.getAbsolutePath.norm
     def relpath: Path = f.toPath.relpath
@@ -305,7 +305,7 @@ object ScriptTestEnv {
   //    dist[*]/target/universal/stage, if present
   //    else, SCALA_HOME if defined
   //    else, not defined
-  lazy val envScalaHome =
+  lazy val envScalaHome: String =
     printf("scalacPath: %s\n", scalacPath.norm)
     if scalacPath.isFile then scalacPath.replaceAll("/bin/scalac", "")
     else envOrElse("SCALA_HOME", "not-found").norm
diff --git a/compiler/test/dotty/tools/utils.scala b/compiler/test/dotty/tools/utils.scala
index 14981e001d38..5dd949643f7f 100644
--- a/compiler/test/dotty/tools/utils.scala
+++ b/compiler/test/dotty/tools/utils.scala
@@ -33,7 +33,7 @@ def scriptsDir(path: String): File = {
   dir
 }
 
-extension (f: File) def absPath =
+extension (f: File) def absPath: String =
   f.getAbsolutePath.replace('\\', '/')
 
 extension (str: String) def dropExtension =
diff --git a/compiler/test/dotty/tools/vulpix/ParallelTesting.scala b/compiler/test/dotty/tools/vulpix/ParallelTesting.scala
index 1bb3c6ae5370..b0b64316bfe7 100644
--- a/compiler/test/dotty/tools/vulpix/ParallelTesting.scala
+++ b/compiler/test/dotty/tools/vulpix/ParallelTesting.scala
@@ -353,8 +353,8 @@ trait ParallelTesting extends RunnerOrchestration:
 
     import summaryReport._
 
-    protected final val realStdout = System.out
-    protected final val realStderr = System.err
+    protected final val realStdout: PrintStream = System.out
+    protected final val realStderr: PrintStream = System.err
 
     /** A runnable that logs its contents in a buffer */
     trait LoggedRunnable extends Runnable {
diff --git a/presentation-compiler/src/main/dotty/tools/pc/InferExpectedType.scala b/presentation-compiler/src/main/dotty/tools/pc/InferExpectedType.scala
index 8640f518c0f1..2aecbb7b36b6 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/InferExpectedType.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/InferExpectedType.scala
@@ -26,8 +26,8 @@ class InferExpectedType(
     driver: InteractiveDriver,
     params: OffsetParams
 )(implicit rc: ReportContext):
-  val uri = params.uri().nn
-  val code = params.text().nn
+  val uri: java.net.URI = params.uri()
+  val code: String = params.text()
 
   val sourceFile = SourceFile.virtual(uri, code)
   driver.run(uri, sourceFile)
diff --git a/presentation-compiler/src/main/dotty/tools/pc/PcInlayHintsProvider.scala b/presentation-compiler/src/main/dotty/tools/pc/PcInlayHintsProvider.scala
index 29396a5c0d32..395548822d96 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/PcInlayHintsProvider.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/PcInlayHintsProvider.scala
@@ -39,11 +39,11 @@ class PcInlayHintsProvider(
     symbolSearch: SymbolSearch,
 )(using ReportContext):
 
-  val uri = params.uri().nn
-  val filePath = Paths.get(uri).nn
-  val sourceText = params.text().nn
-  val text = sourceText.toCharArray().nn
-  val source =
+  val uri: java.net.URI = params.uri()
+  val filePath: java.nio.file.Path = Paths.get(uri)
+  val sourceText: String = params.text()
+  val text: Array[Char] = sourceText.toCharArray()
+  val source: SourceFile =
     SourceFile.virtual(filePath.toString, sourceText)
   driver.run(uri, source)
   given InlayHintsParams = params
@@ -119,7 +119,7 @@ class PcInlayHintsProvider(
               InlayHintKind.Type,
             )
             .addDefinition(adjustedPos.start)
-      case Parameters(isInfixFun, args) =>        
+      case Parameters(isInfixFun, args) =>
         def isNamedParam(pos: SourcePosition): Boolean =
           val start = text.indexWhere(!_.isWhitespace, pos.start)
           val end = text.lastIndexWhere(!_.isWhitespace, pos.end - 1)
@@ -142,9 +142,9 @@ class PcInlayHintsProvider(
           case (ih, (name, pos0, isByName)) =>
             val pos = adjustPos(pos0)
             val isBlock = isBlockParam(pos)
-            val namedLabel = 
+            val namedLabel =
               if params.namedParameters() && !isInfixFun && !isBlock && !isNamedParam(pos) then s"${name} = " else ""
-            val byNameLabel = 
+            val byNameLabel =
               if params.byNameParameters() && isByName && (!isInfixFun || isBlock) then "=> " else ""
 
             val labelStr = s"${namedLabel}${byNameLabel}"
@@ -432,19 +432,19 @@ object InferredType:
 end InferredType
 
 object Parameters:
-  def unapply(tree: Tree)(using params: InlayHintsParams, ctx: Context): Option[(Boolean, List[(Name, SourcePosition, Boolean)])] = 
-    def shouldSkipFun(fun: Tree)(using Context): Boolean = 
+  def unapply(tree: Tree)(using params: InlayHintsParams, ctx: Context): Option[(Boolean, List[(Name, SourcePosition, Boolean)])] =
+    def shouldSkipFun(fun: Tree)(using Context): Boolean =
       fun match
         case sel: Select => isForComprehensionMethod(sel) || sel.symbol.name == nme.unapply || sel.symbol.is(Flags.JavaDefined)
         case _ => false
 
-    def isInfixFun(fun: Tree, args: List[Tree])(using Context): Boolean = 
+    def isInfixFun(fun: Tree, args: List[Tree])(using Context): Boolean =
       val isInfixSelect = fun match
         case Select(sel, _) => sel.isInfix
         case _ => false
       val source = fun.source
       if args.isEmpty then isInfixSelect
-      else 
+      else
         (!(fun.span.end until args.head.span.start)
         .map(source.apply)
         .contains('.') && fun.symbol.is(Flags.ExtensionMethod)) || isInfixSelect
@@ -467,7 +467,7 @@ object Parameters:
 
     if (params.namedParameters() || params.byNameParameters()) then
       tree match
-        case Apply(fun, args) if isRealApply(fun) => 
+        case Apply(fun, args) if isRealApply(fun) =>
           val underlyingFun = getUnderlyingFun(fun)
           if shouldSkipFun(underlyingFun) then
             None
@@ -475,7 +475,7 @@ object Parameters:
             val funTp = fun.typeOpt.widenTermRefExpr
             val paramNames = funTp.paramNamess.flatten
             val paramInfos = funTp.paramInfoss.flatten
-            
+
             Some(
               isInfixFun(fun, args) || underlyingFun.isInfix,
               (
@@ -483,7 +483,7 @@ object Parameters:
                 .zip(paramNames)
                 .zip(paramInfos)
                 .collect {
-                  case ((arg, paramName), paramInfo) if !arg.span.isZeroExtent && !isDefaultArg(arg) => 
+                  case ((arg, paramName), paramInfo) if !arg.span.isZeroExtent && !isDefaultArg(arg) =>
                     (paramName.fieldName, arg.sourcePos, paramInfo.isByName)
                 }
               )
diff --git a/presentation-compiler/src/main/dotty/tools/pc/ScalaPresentationCompiler.scala b/presentation-compiler/src/main/dotty/tools/pc/ScalaPresentationCompiler.scala
index 18311d1b7853..81517524e3a6 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/ScalaPresentationCompiler.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/ScalaPresentationCompiler.scala
@@ -119,7 +119,7 @@ case class ScalaPresentationCompiler(
   ): PresentationCompiler =
     copy(completionItemPriority = priority)
 
-  override def withBuildTargetName(buildTargetName: String) =
+  override def withBuildTargetName(buildTargetName: String): PresentationCompiler =
     copy(buildTargetName = Some(buildTargetName))
 
   override def withReportsLoggerLevel(level: String): PresentationCompiler =
diff --git a/presentation-compiler/src/main/dotty/tools/pc/WithCompilationUnit.scala b/presentation-compiler/src/main/dotty/tools/pc/WithCompilationUnit.scala
index 56be6614bbd4..5ba89324fca8 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/WithCompilationUnit.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/WithCompilationUnit.scala
@@ -22,11 +22,11 @@ class WithCompilationUnit(
     val driver: InteractiveDriver,
     params: VirtualFileParams,
 ):
-  val uri = params.uri()
-  val filePath = Paths.get(uri)
-  val sourceText = params.text
-  val text = sourceText.toCharArray()
-  val source =
+  val uri: java.net.URI = params.uri()
+  val filePath: java.nio.file.Path = Paths.get(uri)
+  val sourceText: String = params.text
+  val text: Array[Char] = sourceText.toCharArray()
+  val source: SourceFile =
     SourceFile.virtual(filePath.toString, sourceText)
   driver.run(uri, source)
   given ctx: Context = driver.currentCtx
diff --git a/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala b/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
index ea3565574ce3..61a93dc1bccb 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala
@@ -76,7 +76,7 @@ class Completions(
         if appl.fun == funSel && sel == fun => false
       case _ => true) &&
       (adjustedPath match
-        /* In case of `class X derives TC@@` we shouldn't add `[]` 
+        /* In case of `class X derives TC@@` we shouldn't add `[]`
          */
         case Ident(_) :: (templ: untpd.DerivingTemplate) :: _ =>
           val pos = completionPos.toSourcePosition
@@ -215,9 +215,9 @@ class Completions(
                 case mt@MethodType(termNames) if app.symbol.paramSymss.last.exists(_.is(Given)) &&
                   !text.substring(app.fun.span.start, arg.span.end).contains("using") =>
                   suffix.withNewPrefix(Affix(PrefixKind.Using))
-                case _ => suffix    
+                case _ => suffix
             case _ => suffix
-        
+
       }
       .chain { suffix => // for () suffix
         if shouldAddSuffix && symbol.is(Flags.Method) then
@@ -907,7 +907,7 @@ class Completions(
       application: CompletionApplication
   ): Ordering[CompletionValue] =
     new Ordering[CompletionValue]:
-      val queryLower = completionPos.query.toLowerCase()
+      val queryLower: String = completionPos.query.toLowerCase()
       val fuzzyCache = mutable.Map.empty[CompletionValue, Int]
 
       def compareLocalSymbols(s1: Symbol, s2: Symbol): Int =
diff --git a/presentation-compiler/test/dotty/tools/pc/base/BasePCSuite.scala b/presentation-compiler/test/dotty/tools/pc/base/BasePCSuite.scala
index a26c31ef084d..5128303d105f 100644
--- a/presentation-compiler/test/dotty/tools/pc/base/BasePCSuite.scala
+++ b/presentation-compiler/test/dotty/tools/pc/base/BasePCSuite.scala
@@ -25,7 +25,7 @@ import org.junit.runner.RunWith
 import scala.meta.pc.CompletionItemPriority
 
 object TestResources:
-  val classpath = BuildInfo.ideTestsDependencyClasspath.map(_.toPath).toSeq
+  val classpath: Seq[Path] = BuildInfo.ideTestsDependencyClasspath.map(_.toPath).toSeq
   val classpathSearch =
     ClasspathSearch.fromClasspath(classpath, ExcludedPackagesHandler.default)
 
@@ -34,7 +34,7 @@ abstract class BasePCSuite extends PcAssertions:
   val completionItemPriority: CompletionItemPriority = (_: String) => 0
   private val isDebug = ManagementFactory.getRuntimeMXBean.getInputArguments.toString.contains("-agentlib:jdwp")
 
-  val tmp = Files.createTempDirectory("stable-pc-tests")
+  val tmp: Path = Files.createTempDirectory("stable-pc-tests")
   val executorService: ScheduledExecutorService =
     Executors.newSingleThreadScheduledExecutor()
   val testingWorkspaceSearch = TestingWorkspaceSearch(
diff --git a/presentation-compiler/test/dotty/tools/pc/utils/MockSymbolSearch.scala b/presentation-compiler/test/dotty/tools/pc/utils/MockSymbolSearch.scala
index 459c41e3c8e5..735ee846be2d 100644
--- a/presentation-compiler/test/dotty/tools/pc/utils/MockSymbolSearch.scala
+++ b/presentation-compiler/test/dotty/tools/pc/utils/MockSymbolSearch.scala
@@ -66,7 +66,7 @@ class MockSymbolSearch(
   override def documentation(
       symbol: String,
       parents: ParentSymbols
-  ) = documentation(symbol, parents, ContentType.MARKDOWN)
+  ): Optional[SymbolDocumentation] = documentation(symbol, parents, ContentType.MARKDOWN)
 
   override def documentation(
       symbol: String,
diff --git a/tests/explicit-nulls/warn/expose-flexible-types.check b/tests/explicit-nulls/warn/expose-flexible-types.check
new file mode 100644
index 000000000000..5e0f8519b7e7
--- /dev/null
+++ b/tests/explicit-nulls/warn/expose-flexible-types.check
@@ -0,0 +1,56 @@
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:4:6 --------------------------------------------------
+4 |  def f = s.trim // warn
+  |      ^
+  |   method f exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:8:6 --------------------------------------------------
+8 |  def h = (s.trim, s.length) // warn
+  |      ^
+  |method h exposes a flexible type in its inferred result type ((String)?, Int). Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:9:16 -------------------------------------------------
+9 |  protected def i = s.trim // warn
+  |                ^
+  |   method i exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:11:19 ------------------------------------------------
+11 |  private[foo] def k = s.trim // warn
+   |                   ^
+   | method k exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:12:6 -------------------------------------------------
+12 |  val ss = s.replace("a", "A") // warn
+   |      ^
+   | value ss exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:13:6 -------------------------------------------------
+13 |  val ss2 = Seq(s.trim) // warn
+   |      ^
+   |value ss2 exposes a flexible type in its inferred result type Seq[(String)?]. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:21:6 -------------------------------------------------
+21 |  def f = s2.trim // warn
+   |      ^
+   | method f exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:25:6 -------------------------------------------------
+25 |  def h = (s2.trim, s2.length) // warn
+   |      ^
+   |method h exposes a flexible type in its inferred result type ((String)?, Int). Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:26:6 -------------------------------------------------
+26 |  val ss = s2.replace("a", "A") // warn
+   |      ^
+   | value ss exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:27:6 -------------------------------------------------
+27 |  val ss2 = Seq(s2.trim) // warn
+   |      ^
+   |value ss2 exposes a flexible type in its inferred result type Seq[(String)?]. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:32:4 -------------------------------------------------
+32 |def f = s2.trim // warn
+   |    ^
+   | method f exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:36:4 -------------------------------------------------
+36 |def h = (s2.trim, s2.length) // warn
+   |    ^
+   |method h exposes a flexible type in its inferred result type ((String)?, Int). Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:37:4 -------------------------------------------------
+37 |val ss = s2.replace("a", "A") // warn
+   |    ^
+   | value ss exposes a flexible type in its inferred result type (String)?. Consider annotating the type explicitly
+-- Warning: tests/explicit-nulls/warn/expose-flexible-types.scala:38:4 -------------------------------------------------
+38 |val ss2 = Seq(s2.trim) // warn
+   |    ^
+   |value ss2 exposes a flexible type in its inferred result type Seq[(String)?]. Consider annotating the type explicitly
diff --git a/tests/explicit-nulls/warn/expose-flexible-types.scala b/tests/explicit-nulls/warn/expose-flexible-types.scala
new file mode 100644
index 000000000000..9908e6228cd4
--- /dev/null
+++ b/tests/explicit-nulls/warn/expose-flexible-types.scala
@@ -0,0 +1,41 @@
+package foo
+
+class C(s: String):
+  def f = s.trim // warn
+  def g: String  =
+    val s2 = s.trim
+    s2
+  def h = (s.trim, s.length) // warn
+  protected def i = s.trim // warn
+  private def j = s.trim
+  private[foo] def k = s.trim // warn
+  val ss = s.replace("a", "A") // warn
+  val ss2 = Seq(s.trim) // warn
+  val ss3: Seq[String] =
+    val s3 = s.trim
+    Seq(s3)
+
+val s2: String = ""
+
+object O:
+  def f = s2.trim // warn
+  def g: String  =
+    val s3 = s2.trim
+    s3
+  def h = (s2.trim, s2.length) // warn
+  val ss = s2.replace("a", "A") // warn
+  val ss2 = Seq(s2.trim) // warn
+  val ss3: Seq[String] =
+    val s3 = s2.trim
+    Seq(s3)
+
+def f = s2.trim // warn
+def g: String  =
+  val s3 = s2.trim
+  s3
+def h = (s2.trim, s2.length) // warn
+val ss = s2.replace("a", "A") // warn
+val ss2 = Seq(s2.trim) // warn
+val ss3: Seq[String] =
+  val s3 = s2.trim
+  Seq(s3)
\ No newline at end of file
diff --git a/tests/explicit-nulls/warn/unnecessary-nn.scala b/tests/explicit-nulls/warn/unnecessary-nn.scala
index 0e93b61fb408..82d87e75c0a5 100644
--- a/tests/explicit-nulls/warn/unnecessary-nn.scala
+++ b/tests/explicit-nulls/warn/unnecessary-nn.scala
@@ -9,7 +9,7 @@ def f6[T >: String|Null](s: String|Null): T = s.nn // warn
 def f5a[T <: String](s: T): String = s.nn // warn
 
 // flexible types
-def f7(s: String|Null) = "".concat(s.nn) // warn
+def f7(s: String|Null): String = "".concat(s.nn) // warn
 def f8(s: String): String = s.trim().nn // OK because the .nn could be useful as a dynamic null check