Warn when a non-fully-dependent class is used with applied constructor types

Author: KacperFKorban

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

@@ -224,7 +224,8 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ExtensionHasDefaultID // errorNumber: 208
   case FormatInterpolationErrorID // errorNumber: 209
   case ValueClassCannotExtendAliasOfAnyValID // errorNumber: 210
-  case PointlessAppliedConstructorTypeID // errorNumber: 211
+  case OnlyFullyDependentAppliedConstructorTypeID // errorNumber: 211
+  case PointlessAppliedConstructorTypeID // errorNumber: 212
 
   def errorNumber = ordinal - 1
 

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

@@ -3471,3 +3471,10 @@ final class PointlessAppliedConstructorType(tpt: untpd.Tree, args: List[untpd.Tr
         |If you want to track a precise type of any of the class parameters, make sure to mark the parameter as `tracked`.
         |Otherwise, you can safely remove the argument list from the type.
         |"""
+
+final class OnlyFullyDependentAppliedConstructorType()(using Context)
+  extends TypeMsg(OnlyFullyDependentAppliedConstructorTypeID):
+  override protected def msg(using Context): String =
+    i"Applied constructor type can only be used with classes that only have tracked parameters"
+
+  override protected def explain(using Context): String = ""

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

@@ -1722,6 +1722,10 @@ trait Applications extends Compatibility {
           def apply(tp: Type) = mapOver(tp.widenSkolem)
         val preciseTp = widenSkolemsMap(tree1.tpe)
         val classTp = typedType(tpt).tpe
+        def classSymbolHasOnlyTrackedParameters =
+          classTp.classSymbol.primaryConstructor.paramSymss.flatten.filter(_.isTerm).forall(_.is(Tracked))
+        if !preciseTp.isError && !classSymbolHasOnlyTrackedParameters then
+          report.warning(OnlyFullyDependentAppliedConstructorType(), tree.srcPos)
         if !preciseTp.isError && (preciseTp frozen_=:= classTp) then
           report.warning(PointlessAppliedConstructorType(tpt, tree.args, classTp), tree.srcPos)
         TypeTree(preciseTp)

tests/pos/applied_constructor_types.scala

@@ -37,19 +37,19 @@ object Test extends App {
   val box2: Box(O.InnerClass(n)) = Box(O.InnerClass(n))
   val box3: Box(O.InnerClass(O.m)) = Box(O.InnerClass(O.m))
 
-  val person: Person("Kasia", 27) = Person("Kasia", 27)
-  val person1: Person("Kasia", n) = Person("Kasia", n)
-  val person2: Person("Kasia", O.m) = Person("Kasia", O.m)
+  val person: Person("Kasia", 27) = Person("Kasia", 27) // warn
+  val person1: Person("Kasia", n) = Person("Kasia", n) // warn
+  val person2: Person("Kasia", O.m) = Person("Kasia", O.m) // warn
 
-  val personPrime: PersonPrime("Kasia")(27) = PersonPrime("Kasia")(27)
-  val personPrime1: PersonPrime("Kasia")(n) = PersonPrime("Kasia")(n)
-  val personPrime2: PersonPrime("Kasia")(O.m) = PersonPrime("Kasia")(O.m)
+  val personPrime: PersonPrime("Kasia")(27) = PersonPrime("Kasia")(27) // warn
+  val personPrime1: PersonPrime("Kasia")(n) = PersonPrime("Kasia")(n) // warn
+  val personPrime2: PersonPrime("Kasia")(O.m) = PersonPrime("Kasia")(O.m) // warn
 
-  val personBis: PersonBis("Kasia")(27) = PersonBis("Kasia")(27)
-  val personBis1: PersonBis("Kasia")(n) = PersonBis("Kasia")(n)
-  val personBis2: PersonBis("Kasia")(O.m) = PersonBis("Kasia")(O.m)
+  val personBis: PersonBis("Kasia")(27) = PersonBis("Kasia")(27) // warn
+  val personBis1: PersonBis("Kasia")(n) = PersonBis("Kasia")(n) // warn
+  val personBis2: PersonBis("Kasia")(O.m) = PersonBis("Kasia")(O.m) // warn
 
-  val generic1: Generic(compiletime.erasedValue[Int]) = Generic(42)
-  val generic2: Generic(??? : Int) = Generic(42)
-  val generic3: Generic(43) = Generic(42)
+  val generic1: Generic(compiletime.erasedValue[Int]) = Generic(42) // warn
+  val generic2: Generic(??? : Int) = Generic(42) // warn
+  val generic3: Generic(43) = Generic(42) // warn
 }

tests/warn/applied_constructor_types.check

@@ -1,7 +1,4 @@
--- [E208] Type Warning: tests/warn/applied_constructor_types.scala:6:10 ------------------------------------------------
+-- [E209] Type Warning: tests/warn/applied_constructor_types.scala:6:10 ------------------------------------------------
 6 |  val v1: UnspecificBox(4) = UnspecificBox(4) // warn
   |          ^^^^^^^^^^^^^^^^
-  |          Applied constructor type UnspecificBox(4) has no effect.
-  |          The resulting type of UnspecificBox(4) is the same as its base type, namely: UnspecificBox
-  |
-  | longer explanation available when compiling with `-explain`
+  |          Applied constructor type can only be used with classes that only have tracked parameters