Add a warning message for a pointless applied constructor type use

Author: KacperFKorban

Diff for: compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -221,6 +221,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case GivenSearchPriorityID // errorNumber: 205
   case EnumMayNotBeValueClassesID // errorNumber: 206
   case IllegalUnrollPlacementID // errorNumber: 207
+  case PointlessAppliedConstructorTypeID // errorNumber: 208
 
   def errorNumber = ordinal - 1
 

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3433,3 +3433,17 @@ extends DeclarationMsg(IllegalUnrollPlacementID):
 
   def explain(using Context) = ""
 end IllegalUnrollPlacement
+
+final class PointlessAppliedConstructorType(tpt: untpd.Tree, args: List[untpd.Tree], tpe: Type)(using Context) extends TypeMsg(PointlessAppliedConstructorTypeID):
+  override protected def msg(using Context): String =
+    val act = i"$tpt(${args.map(_.show).mkString(", ")})"
+    i"""|Applied constructor type $act has no effect.
+        |The resulting type of $act is the same as its base type, namely: $tpe""".stripMargin
+
+  override protected def explain(using Context): String =
+    i"""|Applied constructor types are used to ascribe specialized types of constructor applications.
+        |To benefit from this feature, the constructor in question has to have a more specific type than the class itself.
+        |
+        |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.
+        |"""

Diff for: compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1701,8 +1701,8 @@ trait Applications extends Compatibility {
       def apply(tp: Type) = mapOver(tp.widenSkolem)
     val preciseTp = widenSkolemsMap(tree1.tpe)
     val classTp = typedType(tpt).tpe
-    if preciseTp frozen_=:= classTp then
-      report.warning(em"Blop blop")
+    if !preciseTp.isError && (preciseTp frozen_=:= classTp) then
+      report.warning(PointlessAppliedConstructorType(tpt, tree.args, classTp), tree.srcPos)
     TypeTree(preciseTp)
 
   /** Is given method reference applicable to argument trees `args`?

Diff for: tests/neg/applied_constructor_types.check

@@ -4,18 +4,15 @@
   |          Not found: type f
   |
   | longer explanation available when compiling with `-explain`
-Blop blop
 -- [E006] Not Found Error: tests/neg/applied_constructor_types.scala:9:10 ----------------------------------------------
 9 |  val v2: id(1) = f(1) // error
   |          ^^
   |          Not found: type id - did you mean is?
   |
   | longer explanation available when compiling with `-explain`
-Blop blop
 -- [E006] Not Found Error: tests/neg/applied_constructor_types.scala:10:10 ---------------------------------------------
 10 |  val v3: idDependent(1) = f(1) // error
    |          ^^^^^^^^^^^
    |          Not found: type idDependent
    |
    | longer explanation available when compiling with `-explain`
-Blop blop

Diff for: tests/warn/applied_constructor_types.check

@@ -1 +1,7 @@
-TODO
+-- [E208] 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`