Take @TargetNAME into account when resolving extension methods (#16487)

Take @TargetNAME into account when resolving extension methods of value
classes

Before target name we only matched on signatures. This was OK, since
multiple extension methods of the same class must be different,
otherwise we will get a "have the same erasure" error later at
erasurePhase. But with @TargetNAME that's now a legal situation that
needs to be resolved correctly. We do this by propagating the target
name to the extension method and verifying that the target names of the
original and extension methods match.

Fixes #16464

Author: smarter

compiler/src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -188,9 +188,17 @@ object ExtensionMethods {
       val companion = imeth.owner.companionModule
       val companionInfo = companion.info
       val candidates = companionInfo.decl(extensionName(imeth)).alternatives
-      val matching =
-        // See the documentation of `memberSignature` to understand why `.stripPoly.ensureMethodic` is needed here.
-        candidates filter (c => FullParameterization.memberSignature(c.info) == imeth.info.stripPoly.ensureMethodic.signature)
+      def matches(candidate: SingleDenotation) =
+        FullParameterization.memberSignature(candidate.info) == imeth.info.stripPoly.ensureMethodic.signature
+          // See the documentation of `memberSignature` to understand why `.stripPoly.ensureMethodic` is needed here.
+        && (if imeth.targetName == imeth.name then
+              // imeth does not have a @targetName annotation, candidate should not have one either
+              candidate.symbol.targetName == candidate.symbol.name
+            else
+              // imeth has a @targetName annotation, candidate's target name must match
+              imeth.targetName == candidate.symbol.targetName
+           )
+      val matching = candidates.filter(matches)
       assert(matching.nonEmpty,
        i"""no extension method found for:
           |
@@ -203,6 +211,9 @@ object ExtensionMethods {
           | Candidates (signatures normalized):
           |
           | ${candidates.map(c => s"${c.name}:${c.info.signature}:${FullParameterization.memberSignature(c.info)}").mkString("\n")}""")
+      if matching.tail.nonEmpty then
+        // this case will report a "have the same erasure" error later at erasure pahse
+        report.log(i"mutiple extension methods match $imeth: ${candidates.map(c => i"${c.name}:${c.info}")}")
       matching.head.symbol.asTerm
     }
 }

tests/neg/i16464.scala

@@ -0,0 +1,6 @@
+
+implicit final class SomeOps(e: Int) extends AnyVal:
+  def -(other: Seq[Int]) = List(1)
+  def -(other: Seq[Long]) = List(2) // error: double definition
+
+def main(): Unit = 1 - Seq.empty[Int]

tests/pos/i16464.scala

@@ -0,0 +1,26 @@
+import scala.annotation.targetName
+
+object test1:
+  implicit final class SomeOps(e: Int) extends AnyVal:
+    @targetName("a")
+    def -(other: Seq[Int]) = List(1)
+    @targetName("b")
+    def -(other: Seq[Long]) = List(2)
+
+  def main(): Unit = 1 - Seq.empty[Int]
+
+object test2:
+  implicit final class SomeOps(e: Int) extends AnyVal:
+    @targetName("a")
+    def -(other: Seq[Int]) = List(1)
+    def -(other: Seq[Long]) = List(2)
+
+  def main(): Unit = 1 - Seq.empty[Int]
+
+object test3:
+  implicit final class SomeOps(e: Int) extends AnyVal:
+    def -(other: Seq[Int]) = List(1)
+    @targetName("b")
+    def -(other: Seq[Long]) = List(2)
+
+  def main(): Unit = 1 - Seq.empty[Int]

tests/run/i16464.scala

@@ -0,0 +1,10 @@
+import scala.annotation.targetName
+
+implicit final class SomeOps(e: Int) extends AnyVal:
+  @targetName("a")
+  def -(other: Seq[Int]) = List(1)
+  @targetName("b")
+  def -(other: Seq[Long]) = List(2)
+
+@main
+def Test(): Unit = 1 - Seq.empty[Int]