Tweak ExtensionNullifiedByMember (#22268)

The warning accounted for an opaque receiver but not for opaque
parameter types.

This commit warns only if corresponding parameters are both opaque (or
both transparent).

The warning message about extensions that will _never_ be selected has
limited generality.

This commit addresses conversions to the receiver type.

Maybe the correct fix is "never say never".

Enhances the message noticed at #22267
Fixes #22279

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

@@ -2504,12 +2504,15 @@ class ExtensionNullifiedByMember(method: Symbol, target: Symbol)(using Context)
   extends Message(ExtensionNullifiedByMemberID):
   def kind = MessageKind.PotentialIssue
   def msg(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 and compatible parameter types."""
+    val targetName = hl(target.name.toString)
+    i"""Extension method ${hl(method.name.toString)} will never be selected from type $targetName
+       |because $targetName already has a member with the same name and compatible parameter types."""
   def explain(using Context) =
-    i"""An extension method can be invoked as a regular method, but if that is intended,
+    i"""Although extensions can be overloaded, they do not overload existing member methods.
+       |An extension method can be invoked as a regular method, but if that is the intended usage,
        |it should not be defined as an extension.
-       |Although extensions can be overloaded, they do not overload existing member methods."""
+       |
+       |The extension may be invoked as though selected from an arbitrary type if conversions are in play."""
 
 class TraitCompanionWithMutableStatic()(using Context)
   extends SyntaxMsg(TraitCompanionWithMutableStaticID) {

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

@@ -1156,6 +1156,11 @@ object RefChecks {
    *
    *  If the extension method is nullary, it is always hidden by a member of the same name.
    *  (Either the member is nullary, or the reference is taken as the eta-expansion of the member.)
+   *
+   *  This check is in lieu of a more expensive use-site check that an application failed to use an extension.
+   *  That check would account for accessibility and opacity. As a limitation, this check considers
+   *  only public members, a target receiver that is not an alias, and corresponding method parameters
+   *  that are either both opaque types or both not.
    */
   def checkExtensionMethods(sym: Symbol)(using Context): Unit =
     if sym.is(Extension) && !sym.nextOverriddenSymbol.exists then
@@ -1179,7 +1184,9 @@ object RefChecks {
               val memberParamTps = member.info.stripPoly.firstParamTypes
               !memberParamTps.isEmpty
               && memberParamTps.lengthCompare(paramTps) == 0
-              && memberParamTps.lazyZip(paramTps).forall((m, x) => x frozen_<:< m)
+              && memberParamTps.lazyZip(paramTps).forall: (m, x) =>
+                m.typeSymbol.denot.isOpaqueAlias == x.typeSymbol.denot.isOpaqueAlias
+                && (x frozen_<:< m)
             }
           }
         .exists

tests/warn/i16743.check

@@ -1,84 +1,84 @@
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:30:6 --------------------------------------------------------
 30 |  def t = 27 // warn
    |      ^
-   |      Extension method t will never be selected
+   |      Extension method t will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:32:6 --------------------------------------------------------
 32 |  def g(x: String)(i: Int): String = x*i // warn
    |      ^
-   |      Extension method g will never be selected
+   |      Extension method g will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:33:6 --------------------------------------------------------
 33 |  def h(x: String): String = x // warn
    |      ^
-   |      Extension method h will never be selected
+   |      Extension method h will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:35:6 --------------------------------------------------------
 35 |  def j(x: Any, y: Int): String = (x.toString)*y // warn
    |      ^
-   |      Extension method j will never be selected
+   |      Extension method j will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:36:6 --------------------------------------------------------
 36 |  def k(x: String): String = x // warn
    |      ^
-   |      Extension method k will never be selected
+   |      Extension method k will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:38:6 --------------------------------------------------------
 38 |  def m(using String): String = "m" + summon[String] // warn
    |      ^
-   |      Extension method m will never be selected
+   |      Extension method m will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:39:6 --------------------------------------------------------
 39 |  def n(using String): String = "n" + summon[String] // warn
    |      ^
-   |      Extension method n will never be selected
+   |      Extension method n will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:40:6 --------------------------------------------------------
 40 |  def o: String = "42" // warn
    |      ^
-   |      Extension method o will never be selected
+   |      Extension method o will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:41:6 --------------------------------------------------------
 41 |  def u: Int = 27 // warn
    |      ^
-   |      Extension method u will never be selected
+   |      Extension method u will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:44:6 --------------------------------------------------------
 44 |  def at: Int = 42 // warn
    |      ^
-   |      Extension method at will never be selected
+   |      Extension method at will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:46:6 --------------------------------------------------------
 46 |  def x(using String)(n: Int): Int = summon[String].toInt + n // warn
    |      ^
-   |      Extension method x will never be selected
+   |      Extension method x will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`
 -- [E194] Potential Issue Warning: tests/warn/i16743.scala:47:6 --------------------------------------------------------
 47 |  def y(using String)(s: String): String = s + summon[String] // warn
    |      ^
-   |      Extension method y will never be selected
+   |      Extension method y will never be selected from type T
    |      because T already has a member with the same name and compatible parameter types.
    |
    | longer explanation available when compiling with `-explain`

tests/warn/i22267.check

@@ -0,0 +1,14 @@
+-- [E194] Potential Issue Warning: tests/warn/i22267.scala:13:26 -------------------------------------------------------
+13 |  extension (self: C) def m(n: Double): Unit = println(2->n) // warn
+   |                          ^
+   |                          Extension method m will never be selected from type C
+   |                          because C already has a member with the same name and compatible parameter types.
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | Although extensions can be overloaded, they do not overload existing member methods.
+   | An extension method can be invoked as a regular method, but if that is the intended usage,
+   | it should not be defined as an extension.
+   |
+   | The extension may be invoked as though selected from an arbitrary type if conversions are in play.
+    --------------------------------------------------------------------------------------------------------------------

tests/warn/i22267.scala

@@ -0,0 +1,28 @@
+//> using options -explain
+
+import language.implicitConversions
+
+case class M[T](value: T)
+given [T]: Conversion[M[T], T] = _.value
+class C:
+  def m(n: Double): Unit = println(0->n)
+object C:
+  given Ops = Ops()
+class Ops:
+  extension (self: C) def m(n: Int): Unit = println(1->n)
+  extension (self: C) def m(n: Double): Unit = println(2->n) // warn
+  extension (self: C) def m(s: String): Unit = println(3->s)
+
+@main def test() =
+  val c = M(C())
+  def i = 42
+  def pi = 3.14
+  c.value.m(i)
+  c.value.m(pi)
+  c.m(i) // conversion
+  c.m(pi) // conversion
+  c.m("hello, world") // extension
+  //m(c)(pi)
+  val c0 = C()
+  c0.m(pi)
+  c0.m("hello, world")

tests/warn/i22279.scala

@@ -0,0 +1,24 @@
+
+import scala.io.Source
+
+object Lengths:
+  opaque type Length = Int
+  object Length:
+    def apply(i: Int): Length = i
+  extension (source: Source)
+    def take(length: Length): IndexedSeq[Char] = // no warn
+      source.take(length).to(IndexedSeq)
+end Lengths
+
+trait Taken:
+  def take(n: Lengths.Length): Taken = ???
+
+object Lengthy:
+  import Lengths.*
+  extension (taken: Taken) def take(n: Length): Taken = ??? // warn
+
+@main def test() = println:
+  import Lengths.*
+  val src = Source.fromString("hello, world")
+  val len = Length("hello".length)
+  src.take(len)