Refix avoid GADT casting with ProtoTypes (#18085)

Revert #17755, refix #15867, fixes  #18062

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

@@ -1122,33 +1122,17 @@ trait Implicits:
             adapt(generated, pt.widenExpr, locked)
         else {
           def untpdGenerated = untpd.TypedSplice(generated)
-          def conversionResultType(info: Type): Type = info match
-            case info: PolyType => conversionResultType(info.resType)
-            case info: MethodType if info.isImplicitMethod => conversionResultType(info.resType)
-            case _ =>
-              if info.derivesFrom(defn.ConversionClass) then
-                pt match
-                  case selProto: SelectionProto =>
-                    // we want to avoid embedding a SelectionProto in a Conversion, as the result type
-                    // as it might end up within a GADT cast type, e.g. tests/pos/i15867.scala
-                    // so, if we can find the target result type - as in,
-                    // if it matches the selection prototype, then let's adapt to that instead
-                    // otherwise just skip adapting with a prototype (by returning NoType)
-                    info.baseType(defn.ConversionClass) match
-                      case AppliedType(_, List(_, restpe)) if selProto.isMatchedBy(restpe) =>
-                        restpe
-                      case _ => NoType // can't find conversion result type, avoid adapting with SelectionProto
-                  case _: ProtoType => NoType // avoid adapting with ProtoType
-                  case _            => pt     // not a ProtoType, so use it for adapting
-              else NoType // not a Conversion, don't adapt
+          def producesConversion(info: Type): Boolean = info match
+            case info: PolyType => producesConversion(info.resType)
+            case info: MethodType if info.isImplicitMethod => producesConversion(info.resType)
+            case _ => info.derivesFrom(defn.ConversionClass)
           def tryConversion(using Context) = {
-            val restpeConv = if ref.symbol.is(Given) then conversionResultType(ref.widenTermRefExpr) else NoType
             val untpdConv =
-              if restpeConv.exists then
+              if ref.symbol.is(Given) && producesConversion(ref.symbol.info) then
                 untpd.Select(
                   untpd.TypedSplice(
                     adapt(generated,
-                      defn.ConversionClass.typeRef.appliedTo(argument.tpe, restpeConv),
+                      defn.ConversionClass.typeRef.appliedTo(argument.tpe, pt),
                       locked)),
                   nme.apply)
               else untpdGenerated

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

@@ -4561,7 +4561,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           AnnotatedType(conj, Annotation(defn.UncheckedStableAnnot, tree.symbol.span))
         else conj
       else pt
-    gadts.println(i"insert GADT cast from $tree to $target")
-    tree.cast(target)
+    if target.existsPart(_.isInstanceOf[ProtoType]) then
+      // we want to avoid embedding a SelectionProto in a Conversion, as the result type
+      // as it might end up within a GADT cast type, e.g. tests/pos/i15867.scala
+      // so we just bail - in that example, a GADT cast will be insert on application, so it compiles.
+      // but tests/pos/i18062.scala is an example with a polymorphic method, which requires type variables to
+      // be applied to the tree and then constrained before they match the prototype.
+      // so rather than try to handle all that before calling adapt, let's just bail on this side.
+      tree
+    else
+      gadts.println(i"insert GADT cast from $tree to $target")
+      tree.cast(target)
   end insertGadtCast
 }

tests/pos/i18062.scala

@@ -0,0 +1,14 @@
+trait CB[X] { def get: X }
+
+trait WrapperConvert[F[_], G[_]]:
+  def conv[X](fx: F[X]): G[X]
+
+object WrapperConvert:
+  implicit def id[F[_]]: WrapperConvert[F, F] = new WrapperConvert[F, F]:
+    def conv[X](fx: F[X]): F[X] = fx
+
+transparent inline given convertX[F[_], X](using wc: WrapperConvert[F, CB]): Conversion[F[X], X] =
+  new Conversion[F[X], X]:
+    def apply(fx: F[X]) = wc.conv(fx).get
+
+def test(cb: CB[Int], x: Int): Int = cb + x