Warn if extension receiver has matching member

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1348,6 +1348,7 @@ object SymDenotations {
      *  inClass <-- find denot.symbol      class C { <-- symbol is here
      *
      *                   site: Subtype of both inClass and C
+     * } <-- balance the brace
      */
     final def matchingDecl(inClass: Symbol, site: Type)(using Context): Symbol = {
       var denot = inClass.info.nonPrivateDecl(name)

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

@@ -195,6 +195,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case MatchTypeScrutineeCannotBeHigherKindedID // errorNumber: 179
   case AmbiguousExtensionMethodID // errorNumber 180
   case UnqualifiedCallToAnyRefMethodID // errorNumber: 181
+  case ExtensionNullifiedByMemberID // errorNumber: 182
 
   def errorNumber = ordinal - 1
 

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

@@ -2298,6 +2298,15 @@ class UnqualifiedCallToAnyRefMethod(stat: untpd.Tree, method: Symbol)(using Cont
        |you intended."""
 }
 
+class ExtensionNullifiedByMember(method: Symbol, target: Symbol)(using Context)
+  extends Message(ExtensionNullifiedByMemberID):
+  def kind = MessageKind.PotentialIssue
+  def msg(using Context) = i"Suspicious extension ${hl(method.name.toString)} is already a member of ${hl(target.name.toString)}"
+  def explain(using Context) =
+    i"""Extension method ${hl(method.name.toString)} will never be selected
+       |because ${hl(target.name.toString)} already has a member with the same name.
+       |It can be called as a regular method, but should probably be defined that way."""
+
 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/Typer.scala

@@ -2486,7 +2486,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         linkConstructorParams(sym, tparamSyms, rhsCtx)
 
     if sym.isInlineMethod then rhsCtx.addMode(Mode.InlineableBody)
-    if sym.is(ExtensionMethod) then rhsCtx.addMode(Mode.InExtensionMethod)
+    if sym.is(ExtensionMethod) then
+      if ctx.phase.isTyper then
+        val ValDef(_, paramTpt, _) = termParamssIn(paramss1).head.head
+        if !paramTpt.symbol.denot.isAliasType && !paramTpt.symbol.denot.isOpaqueAlias then
+          val other = paramTpt.tpe.nonPrivateMember(name)
+          if other.exists && sym.denot.info.resultType.matches(other.info) then
+            report.warning(ExtensionNullifiedByMember(sym, paramTpt.symbol), ddef.srcPos)
+      rhsCtx.addMode(Mode.InExtensionMethod)
     val rhs1 = PrepareInlineable.dropInlineIfError(sym,
       if sym.isScala2Macro then typedScala2MacroBody(ddef.rhs)(using rhsCtx)
       else typedExpr(ddef.rhs, tpt1.tpe.widenExpr)(using rhsCtx))

compiler/test/dotty/tools/scripting/ScriptTestEnv.scala

@@ -218,6 +218,7 @@ object ScriptTestEnv {
     def toUrl: String = Paths.get(absPath).toUri.toURL.toString
 
     // Treat norm paths with a leading '/' as absolute (Windows java.io.File#isAbsolute treats them as relative)
+    @annotation.nowarn // hidden by Path#isAbsolute?
     def isAbsolute = p.norm.startsWith("/") || (isWin && p.norm.secondChar == ":")
   }
 

scaladoc-testcases/src/tests/implicitConversions.scala

@@ -45,7 +45,7 @@ class B {
 class C {
   def extensionInCompanion: String = ???
 }
-
+@annotation.nowarn // extensionInCompanion
 object C {
   implicit def companionConversion(c: C): B = ???
 
@@ -70,4 +70,4 @@ package nested {
   }
 
   class Z
-}
+}

scaladoc-testcases/src/tests/inheritedMembers1.scala

@@ -2,6 +2,7 @@ package tests
 package inheritedMembers1
 
 
+/*<-*/@annotation.nowarn/*->*/
 class A
 {
     def A: String

tests/neg-custom-args/fatal-warnings/i9241.scala

@@ -20,6 +20,7 @@ final class Baz private (val x: Int) extends AnyVal {
 }
 
 extension (x: Int)
+  @annotation.nowarn
   def unary_- : Int = ???
   def unary_+[T] : Int = ???
   def unary_!() : Int = ??? // error

tests/neg/i16743.check

@@ -0,0 +1,11 @@
+-- [E182] Potential Issue Error: tests/neg/i16743.scala:7:21 -----------------------------------------------------------
+7 |extension (x: T) def t = 27 // error
+  |                 ^^^^^^^^^^
+  |                 Suspicious extension t is already a member of T
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Extension method t will never be selected
+  | because T already has a member with the same name.
+  | It can be called as a regular method, but should probably be defined that way.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg/i16743.scala

@@ -0,0 +1,24 @@
+
+// scalac: -Werror -explain
+
+trait T:
+  def t = 42
+
+extension (x: T) def t = 27 // error
+
+@main def test() =
+  val x = new T {}
+  println:
+    x.t
+
+trait Foo:
+  type X
+  extension (x: X) def t: Int
+
+trait Bar extends Foo:
+  type X = T
+  extension (x: X) def t = x.t
+
+opaque type IArray[+T] = Array[_ <: T]
+object IArray:
+  extension (arr: IArray[Byte]) def length: Int = arr.asInstanceOf[Array[Byte]].length