check if package names will be encoded in desugar

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -8,7 +8,7 @@ import Symbols._, StdNames._, Trees._, ContextOps._
 import Decorators._, transform.SymUtils._
 import NameKinds.{UniqueName, EvidenceParamName, DefaultGetterName}
 import typer.{Namer, Checking}
-import util.{Property, SourceFile, SourcePosition}
+import util.{Property, SourceFile, SourcePosition, Chars}
 import config.Feature.{sourceVersion, migrateTo3, enabled}
 import config.SourceVersion._
 import collection.mutable.ListBuffer
@@ -521,9 +521,9 @@ object desugar {
         val enumCompanionRef = TermRefTree()
         val enumImport =
           Import(enumCompanionRef, enumCases.flatMap(caseIds).map(
-            enumCase => 
+            enumCase =>
               ImportSelector(enumCase.withSpan(enumCase.span.startPos))
-            ) 
+            )
           )
         (enumImport :: enumStats, enumCases, enumCompanionRef)
       }
@@ -834,7 +834,8 @@ object desugar {
     val impl = mdef.impl
     val mods = mdef.mods
     val moduleName = normalizeName(mdef, impl).asTermName
-    if (mods.is(Package))
+    if mods.is(Package) then
+      checkPackageName(mdef)
       PackageDef(Ident(moduleName),
         cpy.ModuleDef(mdef)(nme.PACKAGE, impl).withMods(mods &~ Package) :: Nil)
     else
@@ -950,6 +951,26 @@ object desugar {
     else tree
   }
 
+  def checkPackageName(mdef: ModuleDef | PackageDef)(using Context): Unit =
+
+    def check(name: Name, errSpan: Span): Unit = name match
+      case name: SimpleName if !errSpan.isSynthetic && name.exists(Chars.willBeEncoded) =>
+        report.warning(em"The package name `$name` will be encoded on the classpath, and can lead to undefined behaviour.", mdef.source.atSpan(errSpan))
+      case _ =>
+
+    def loop(part: RefTree): Unit = part match
+      case part @ Ident(name) => check(name, part.span)
+      case part @ Select(qual: RefTree, name) =>
+        check(name, part.nameSpan)
+        loop(qual)
+      case _ =>
+
+    mdef match
+      case pdef: PackageDef => loop(pdef.pid)
+      case mdef: ModuleDef if mdef.mods.is(Package) => check(mdef.name, mdef.nameSpan)
+      case _ =>
+  end checkPackageName
+
   /** The normalized name of `mdef`. This means
    *   1. Check that the name does not redefine a Scala core class.
    *      If it does redefine, issue an error and return a mangled name instead
@@ -1134,7 +1155,7 @@ object desugar {
       val matchExpr =
         if (tupleOptimizable) rhs
         else
-          val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids)) 
+          val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
           Match(makeSelector(rhs, MatchCheck.IrrefutablePatDef), caseDef :: Nil)
       vars match {
         case Nil if !mods.is(Lazy) =>
@@ -1155,11 +1176,11 @@ object desugar {
           val restDefs =
             for (((named, tpt), n) <- vars.zipWithIndex if named.name != nme.WILDCARD)
             yield
-              if mods.is(Lazy) then 
+              if mods.is(Lazy) then
                 DefDef(named.name.asTermName, Nil, tpt, selector(n))
                   .withMods(mods &~ Lazy)
                   .withSpan(named.span)
-              else 
+              else
                 valDef(
                   ValDef(named.name.asTermName, tpt, selector(n))
                     .withMods(mods)
@@ -1321,6 +1342,7 @@ object desugar {
    *     (i.e. objects having the same name as a wrapped type)
    */
   def packageDef(pdef: PackageDef)(using Context): PackageDef = {
+    checkPackageName(pdef)
     val wrappedTypeNames = pdef.stats.collect {
       case stat: TypeDef if isTopLevelDef(stat) => stat.name
     }

compiler/src/dotty/tools/dotc/util/Chars.scala

@@ -93,4 +93,7 @@ object Chars {
          '|' | '/' | '\\' => true
     case c => isSpecial(c)
   }
+
+  /** Would the character be encoded by `NameTransformer.encode`? */
+  def willBeEncoded(c : Char) : Boolean = !JCharacter.isJavaIdentifierPart(c)
 }

tests/neg-custom-args/fatal-warnings/symbolic-packages.check

@@ -0,0 +1,16 @@
+-- Error: tests/neg-custom-args/fatal-warnings/symbolic-packages.scala:1:8 ---------------------------------------------
+1 |package `with spaces` { // error
+  |        ^^^^^^^^^^^^^
+  |        The package name `with spaces` will be encoded on the classpath, and can lead to undefined behaviour.
+-- Error: tests/neg-custom-args/fatal-warnings/symbolic-packages.scala:5:10 --------------------------------------------
+5 |package +.* { // error // error
+  |          ^
+  |          The package name `*` will be encoded on the classpath, and can lead to undefined behaviour.
+-- Error: tests/neg-custom-args/fatal-warnings/symbolic-packages.scala:5:8 ---------------------------------------------
+5 |package +.* { // error // error
+  |        ^
+  |        The package name `+` will be encoded on the classpath, and can lead to undefined behaviour.
+-- Error: tests/neg-custom-args/fatal-warnings/symbolic-packages.scala:9:16 --------------------------------------------
+9 |package object `mixed_*` { // error
+  |                ^^^^^^^
+  |                The package name `mixed_*` will be encoded on the classpath, and can lead to undefined behaviour.

tests/neg-custom-args/fatal-warnings/symbolic-packages.scala

@@ -0,0 +1,11 @@
+package `with spaces` { // error
+  class Foo
+}
+
+package +.* { // error // error
+  class Bar
+}
+
+package object `mixed_*` { // error
+  class Baz
+}

tests/pos-special/fatal-warnings/stats-in-empty-pkg.scala

@@ -0,0 +1,4 @@
+def foo = 23
+val bar = foo
+var baz = bar
+type Qux = Int