Fix #24711: Use dot notation for Java nested types in error messages

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -513,14 +513,29 @@ class PlainPrinter(_ctx: Context) extends Printer {
   def toTextPrefixOf(tp: NamedType): Text = controlled {
       homogenize(tp.prefix) match {
         case NoPrefix => ""
-        case tp: SingletonType => toTextRef(tp) ~ "."
-        case tp => trimPrefix(toTextLocal(tp)) ~ "#"
+        case prefix: SingletonType => toTextRef(prefix) ~ "."
+        case prefix =>
+          // Use "." for Java nested classes (e.g., java.util.Map.Entry)
+          // Use "#" for Scala type projections (e.g., Outer#Inner)
+          val separator = if (isJavaNestedClass(tp)) "." else "#"
+          trimPrefix(toTextLocal(prefix)) ~ separator
       }
   }
 
   protected def isEmptyPrefix(sym: Symbol): Boolean =
     sym.isEffectiveRoot || sym.isAnonymousClass || sym.name.isReplWrapperName
 
+  /** Check if tp represents a Java nested class that should use "." separator. */
+  protected def isJavaNestedClass(tp: NamedType)(using Context): Boolean = {
+    val sym = tp.symbol
+    sym.exists &&
+    sym.is(JavaDefined) &&
+    sym.isClass &&
+    sym.owner.exists &&
+    sym.owner.is(JavaDefined) &&
+    sym.owner.isClass
+  }
+
   /** String representation of a definition's type following its name,
    *  if symbol is completed, ": ?" otherwise.
    */

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -488,7 +488,14 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         else if (name.isTypeName) typeText(txt)
         else txt
       case tree @ Select(qual, name) =>
-        if (qual.isType) toTextLocal(qual) ~ "#" ~ typeText(toText(name))
+        if (qual.isType) {
+          // Use "." for Java nested classes, "#" for Scala type projections
+          val separator = tree.typeOpt match {
+            case tp: NamedType if isJavaNestedClass(tp) => "."
+            case _ => "#"
+          }
+          toTextLocal(qual) ~ separator ~ typeText(toText(name))
+        }
         else toTextLocal(qual) ~ ("." ~ nameIdText(tree) `provided` (name != nme.CONSTRUCTOR || printDebug))
       case tree: This =>
         optDotPrefix(tree) ~ keywordStr("this") ~ idText(tree)

tests/neg/i24711-java-nested-types.check

@@ -0,0 +1,21 @@
+-- [E007] Type Mismatch Error: tests/neg/i24711-java-nested-types.scala:2:20 -------------------------------------------
+2 |  val test1: Int = (??? : java.util.Map.Entry[String, Int])  // error
+  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                    Found:    java.util.Map.Entry[String, Int]
+  |                    Required: Int
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/i24711-java-nested-types.scala:4:38 -------------------------------------------
+4 |  val test2: Int = java.util.Map.entry("key", 1)  // error
+  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                   Found:    java.util.Map.Entry[String, Int]
+  |                   Required: Int
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/i24711-java-nested-types.scala:7:20 -------------------------------------------
+7 |  val test4: Int = (??? : Outer#Inner)  // error
+  |                    ^^^^^^^^^^^^^^^^^
+  |                    Found:    Test.this.Outer#Inner
+  |                    Required: Int
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i24711-java-nested-types.scala

@@ -0,0 +1,8 @@
+class Test {
+  val test1: Int = (??? : java.util.Map.Entry[String, Int])  // error
+
+  val test2: Int = java.util.Map.entry("key", 1)  // error
+
+  trait Outer { type Inner }
+  val test4: Int = (??? : Outer#Inner)  // error
+}