Warn on unqualified top-level calls to Any or AnyRef methods

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -194,6 +194,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case MissingArgumentListID // errorNumber: 178
   case MatchTypeScrutineeCannotBeHigherKindedID // errorNumber: 179
   case AmbiguousExtensionMethodID // errorNumber 180
+  case UnqualifiedCallToAnyRefMethodID // errorNumber: 181
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2284,6 +2284,16 @@ class PureExpressionInStatementPosition(stat: untpd.Tree, val exprOwner: Symbol)
         |It can be removed without changing the semantics of the program. This may indicate an error."""
 }
 
+class UnqualifiedCallToAnyRefMethod(stat: untpd.Tree, method: Symbol)(using Context)
+  extends Message(UnqualifiedCallToAnyRefMethodID) {
+  def kind = MessageKind.PotentialIssue
+  def msg(using Context) = i"Suspicious top-level unqualified call to ${hl(method.name.toString)}"
+  def explain(using Context) =
+    i"""Top-level unqualified calls to ${hl("AnyRef")} or ${hl("Any")} methods such as ${hl(method.name.toString)} are
+       |resolved to calls on ${hl("Predef")} or on imported methods. This might not be what
+       |you intended."""
+}
+
 class TraitCompanionWithMutableStatic()(using Context)
   extends SyntaxMsg(TraitCompanionWithMutableStaticID) {
   def msg(using Context) = i"Companion of traits cannot define mutable @static fields"

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -1090,6 +1090,12 @@ object RefChecks {
 
   end checkImplicitNotFoundAnnotation
 
+  def checkAnyRefMethodCall(tree: Tree)(using Context) =
+    if tree.symbol.exists
+       && defn.topClasses.contains(tree.symbol.owner)
+       && (!ctx.owner.enclosingClass.exists || ctx.owner.enclosingClass.isPackageObject) then
+      report.warning(UnqualifiedCallToAnyRefMethod(tree, tree.symbol), tree)
+
 }
 import RefChecks._
 
@@ -1168,6 +1174,11 @@ class RefChecks extends MiniPhase { thisPhase =>
       report.error(ex, tree.srcPos)
       tree
   }
+
+  override def transformIdent(tree: Ident)(using Context): Tree =
+    checkAnyRefMethodCall(tree)
+    tree
+
 }
 
 /* todo: rewrite and re-enable

compiler/test/dotty/tools/utils.scala

@@ -17,8 +17,10 @@ import scala.util.control.{ControlThrowable, NonFatal}
 
 import dotc.config.CommandLineParser
 
+object Dummy
+
 def scripts(path: String): Array[File] = {
-  val dir = new File(this.getClass.getResource(path).getPath)
+  val dir = new File(Dummy.getClass.getResource(path).getPath)
   assert(dir.exists && dir.isDirectory, "Couldn't load scripts dir")
   dir.listFiles.filter { f =>
     val path = if f.isDirectory then f.getPath + "/" else f.getPath

tests/neg/i17266.check

@@ -0,0 +1,88 @@
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:4:2 ------------------------------------------------------------
+4 |  synchronized { // error
+  |  ^^^^^^^^^^^^
+  |  Suspicious top-level unqualified call to synchronized
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Top-level unqualified calls to AnyRef or Any methods such as synchronized are
+  | resolved to calls on Predef or on imported methods. This might not be what
+  | you intended.
+   ---------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:17:2 -----------------------------------------------------------
+17 |  synchronized { // error
+   |  ^^^^^^^^^^^^
+   |  Suspicious top-level unqualified call to synchronized
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | Top-level unqualified calls to AnyRef or Any methods such as synchronized are
+   | resolved to calls on Predef or on imported methods. This might not be what
+   | you intended.
+    --------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:108:2 ----------------------------------------------------------
+108 |  wait() // error
+    |  ^^^^
+    |  Suspicious top-level unqualified call to wait
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as wait are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:115:2 ----------------------------------------------------------
+115 |  wait() // error
+    |  ^^^^
+    |  Suspicious top-level unqualified call to wait
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as wait are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:121:2 ----------------------------------------------------------
+121 |  wait(10) // error
+    |  ^^^^
+    |  Suspicious top-level unqualified call to wait
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as wait are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:128:2 ----------------------------------------------------------
+128 |  wait(10) // error
+    |  ^^^^
+    |  Suspicious top-level unqualified call to wait
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as wait are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:134:2 ----------------------------------------------------------
+134 |  hashCode() // error
+    |  ^^^^^^^^
+    |  Suspicious top-level unqualified call to hashCode
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as hashCode are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------
+-- [E181] Potential Issue Error: tests/neg/i17266.scala:141:2 ----------------------------------------------------------
+141 |  hashCode() // error
+    |  ^^^^^^^^
+    |  Suspicious top-level unqualified call to hashCode
+    |-------------------------------------------------------------------------------------------------------------------
+    | Explanation (enabled by `-explain`)
+    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    | Top-level unqualified calls to AnyRef or Any methods such as hashCode are
+    | resolved to calls on Predef or on imported methods. This might not be what
+    | you intended.
+     -------------------------------------------------------------------------------------------------------------------

tests/neg/i17266.scala

@@ -0,0 +1,144 @@
+// scalac: -Werror -explain
+
+def test1 =
+  synchronized { // error
+    println("hello")
+  }
+
+def test2 =
+  this.synchronized { // not an error (should be?)
+    println("hello")
+  }
+
+object MyLib
+
+def test3 =
+  import MyLib.*
+  synchronized { // error
+    println("hello")
+  }
+
+def test4 =
+  1.synchronized { // not an error (should be?)
+    println("hello")
+  }
+
+object Test4:
+  synchronized { // not an error
+    println("hello")
+  }
+
+object Test5:
+  def test5 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+object Test6:
+  import MyLib.*
+  synchronized { // not an error
+    println("hello")
+  }
+
+object Test7:
+  import MyLib.*
+  def test7 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+/*
+object Test7b:
+  def test8 =
+    import MyLib.*
+    synchronized { // already an error: Reference to synchronized is ambiguous.
+      println("hello")
+    }
+*/
+
+class Test8:
+  synchronized { // not an error
+    println("hello")
+  }
+
+class Test9:
+  def test5 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+class Test10:
+  import MyLib.*
+  synchronized { // not an error
+    println("hello")
+  }
+
+class Test11:
+  import MyLib.*
+  def test7 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+trait Test12:
+  synchronized { // not an error
+    println("hello")
+  }
+
+trait Test13:
+  def test5 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+trait Test14:
+  import MyLib.*
+  synchronized { // not an error
+    println("hello")
+  }
+
+trait Test15:
+  import MyLib.*
+  def test7 =
+    synchronized { // not an error
+      println("hello")
+    }
+
+def test16 =
+  wait() // error
+
+def test17 =
+  this.wait() // not an error (should be?)
+
+def test18 =
+  import MyLib.*
+  wait() // error
+
+def test19 =
+  1.wait() // not an error (should be?)
+
+def test20 =
+  wait(10) // error
+
+def test21 =
+  this.wait(10) // not an error (should be?)
+
+def test22 =
+  import MyLib.*
+  wait(10) // error
+
+def test23 =
+  1.wait(10) // not an error (should be?)
+
+def test24 =
+  hashCode() // error
+
+def test25 =
+  this.hashCode() // not an error (should be?)
+
+def test26 =
+  import MyLib.*
+  hashCode() // error
+
+def test27 =
+  1.hashCode()// not an error (should be? probably not)