Fix implicit conversion type resolve in completions

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -384,9 +384,10 @@ object Completion {
       if qual.tpe.isExactlyNothing || qual.tpe.isNullType then
         Map.empty
       else
-        val membersFromConversion =
-          implicitConversionTargets(qual)(using ctx.fresh.setExploreTyperState()).flatMap(accessibleMembers)
-        membersFromConversion.toSeq.groupByName
+        implicitConversionTargets(qual)(using ctx.fresh.setExploreTyperState())
+          .flatMap(accessibleMembers)
+          .toSeq
+          .groupByName
 
     /** Completions from extension methods */
     private def extensionCompletions(qual: Tree)(using Context): CompletionMap =
@@ -492,15 +493,17 @@ object Completion {
 
     /**
      * Given `qual` of type T, finds all the types S such that there exists an implicit conversion
-     * from T to S.
+     * from T to S. It then applies conversion method for proper type parameter resolution.
      *
      * @param qual The argument to which the implicit conversion should be applied.
-     * @return The set of types that `qual` can be converted to.
+     * @return The set of types after `qual` implicit conversion.
      */
     private def implicitConversionTargets(qual: Tree)(using Context): Set[Type] = {
       val typer = ctx.typer
       val conversions = new typer.ImplicitSearch(defn.AnyType, qual, pos.span).allImplicits
-      val targets = conversions.map(_.widen.finalResultType)
+      val convertedTrees = conversions.flatMap(typer.tryApplyingImplicitConversion(_, qual))
+      val targets = convertedTrees.map(_.tpe.finalResultType)
+
       interactiv.println(i"implicit conversion targets considered: ${targets.toList}%, %")
       targets
     }

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

@@ -2289,6 +2289,27 @@ trait Applications extends Compatibility {
     catch
       case NonFatal(_) => None
 
+  /** Tries applying conversion method reference to a provided receiver
+   *
+   *  returns converted tree in case of success.
+   *  None is returned if conversion method application fails.
+   */
+  def tryApplyingImplicitConversion(conversionMethodRef: TermRef, receiver: Tree)(using Context): Option[Tree] =
+    val conversionMethodTree = ref(conversionMethodRef, needLoad = false)
+    val newCtx = ctx.fresh.setNewScope.setReporter(new reporting.ThrowingReporter(ctx.reporter))
+
+    try
+      val appliedTree = inContext(newCtx) {
+        typed(untpd.Apply(conversionMethodTree, untpd.TypedSplice(receiver) :: Nil))
+      }
+
+      if appliedTree.tpe.exists && !appliedTree.tpe.isError then
+        Some(appliedTree)
+      else
+        None
+    catch
+      case NonFatal(x) => None
+
   def isApplicableExtensionMethod(methodRef: TermRef, receiverType: Type)(using Context): Boolean =
     methodRef.symbol.is(ExtensionMethod) && !receiverType.isBottomType &&
       tryApplyingExtensionMethod(methodRef, nullLiteral.asInstance(receiverType)).nonEmpty

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -1150,27 +1150,62 @@ class CompletionTest {
       .completion(("map", Method, "[B](f: Int => B): Foo[B]"))
   }
 
-  // This test is not passing due to https://github.com/lampepfl/dotty/issues/14687
-  // @Test def higherKindedMatchTypeImplicitConversionCompletion: Unit = {
-  //   val expected = Set(
-  //     ("mapBoo", Method, "[B](op: Int => B): Boo[B]"),
-  //     ("mapFoo", Method, "[B](op: Int => B): Foo[B]"),
-  //   )
-  //   code"""import scala.language.implicitConversions
-  //         |case class Foo[A](x: A) {
-  //         |  def mapFoo[B](op: A => B): Foo[B] = ???
-  //         |}
-  //         |case class Boo[A](x: A) {
-  //         |  def mapBoo[B](op: A => B): Boo[B] = ???
-  //         |}
-  //         |type M[A] = A match {
-  //         |  case Int => Foo[Int]
-  //         |}
-  //         |implicit def fooToBoo[A](x: Foo[A]): Boo[A] = Boo(x.x)
-  //         |case class Bar[F[_]](bar: F[Int])
-  //         |def foo(x: Bar[M]) = x.bar.m${m1}"""
-  //            .completion(m1, expected)
-  // }
+  @Test def higherKindedMatchTypeImplicitConversionCompletion: Unit = {
+    code"""import scala.language.implicitConversions
+          |case class Foo[A](x: A) {
+          |  def mapFoo[B](op: A => B): Foo[B] = ???
+          |}
+          |case class Boo[A](x: A) {
+          |  def mapBoo[B](op: A => B): Boo[B] = ???
+          |}
+          |type M[A] = A match {
+          |  case Int => Foo[Int]
+          |}
+          |implicit def fooToBoo[A](x: Foo[A]): Boo[A] = Boo(x.x)
+          |case class Bar[F[_]](bar: F[Int])
+          |def foo(x: Bar[M]) = x.bar.m${m1}"""
+      .completion(
+        ("mapBoo", Method, "[B](op: Int => B): Boo[B]"),
+        ("mapFoo", Method, "[B](op: Int => B): Foo[B]")
+      )
+  }
+
+  @Test def higherKindedTypeInferenceTest: Unit = {
+    val expected = Set(
+        ("fooTest", Method, "(x: Int): String")
+      )
+    code"""class Test[A, B] {
+          |  def fooTest(x: A): B = ???
+          |}
+          |
+          |object M:
+          |  val test = new Test[Int, String] {}
+          |  test.foo${m1}
+          |  (new Test[Int, String] {}).foo${m2}"""
+      .completion(m1, expected)
+      .completion(m2, expected)
+  }
+
+  @Test def higherKindedImplicitConversionsCompletions: Unit = {
+    val expected = Set(
+      ("mapBoo", Method, "[B](f: Int => B): Boo[B]"),
+      ("mapFoo", Method, "[B](f: Int => B): Foo[B]"),
+    )
+    code"""import scala.language.implicitConversions
+          |case class Foo[A](x: A) {
+          |  def mapFoo[B](f: A => B): Foo[B] = ???
+          |}
+          |case class Boo[C](x: C) {
+          |  def mapBoo[B](f: C => B): Boo[B] = ???
+          |}
+          |implicit def fooToBoo[D](x: Foo[D]): Boo[D] = Boo(x.x)
+          |object Test:
+          |  val x = Foo(1)
+          |  x.ma${m1}
+          |  Foo(1).ma${m2}"""
+      .completion(m1, expected)
+      .completion(m2, expected)
+  }
 
   @Test def higherKindedMatchTypeExtensionMethodCompletion: Unit = {
     code"""trait Foo[A] {