Add message to dummy capture paramter usages

Author: noti0na1

compiler/src/dotty/tools/dotc/core/SymUtils.scala

@@ -90,6 +90,9 @@ class SymUtils:
     def isContextBoundCompanion(using Context): Boolean =
       self.is(Synthetic) && self.infoOrCompleter.typeSymbol == defn.CBCompanion
 
+    def isDummyCaptureParam(using Context): Boolean =
+      self.is(Synthetic) && self.is(CaptureParam) && !(self.isClass || self.is(Method))
+
     /** Is this a case class for which a product mirror is generated?
     *  Excluded are value classes, abstract classes and case classes with more than one
     *  parameter section.

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

@@ -208,7 +208,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnstableInlineAccessorID // errorNumber: 192
   case VolatileOnValID // errorNumber: 193
   case ExtensionNullifiedByMemberID // errorNumber: 194
-  case ConstructorProxyNotValueID // errorNumber: 195
+  case PhantomSymbolNotValueID // errorNumber: 195
   case ContextBoundCompanionNotValueID // errorNumber: 196
   case InlinedAnonClassWarningID // errorNumber: 197
   case UnusedSymbolID // errorNumber: 198

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

@@ -3277,7 +3277,7 @@ extends SyntaxMsg(VolatileOnValID):
   protected def explain(using Context): String = ""
 
 class ConstructorProxyNotValue(sym: Symbol)(using Context)
-extends TypeMsg(ConstructorProxyNotValueID):
+extends TypeMsg(PhantomSymbolNotValueID):
   protected def msg(using Context): String =
     i"constructor proxy $sym cannot be used as a value"
   protected def explain(using Context): String =
@@ -3292,7 +3292,7 @@ extends TypeMsg(ConstructorProxyNotValueID):
        |but not as a stand-alone value."""
 
 class ContextBoundCompanionNotValue(sym: Symbol)(using Context)
-extends TypeMsg(ConstructorProxyNotValueID):
+extends TypeMsg(PhantomSymbolNotValueID):
   protected def msg(using Context): String =
     i"context bound companion $sym cannot be used as a value"
   protected def explain(using Context): String =
@@ -3311,6 +3311,22 @@ extends TypeMsg(ConstructorProxyNotValueID):
        |companion value with the (term-)name `A`. However, these context bound companions
        |are not values themselves, they can only be referred to in selections."""
 
+class DummyCaptureParamNotValue(sym: Symbol)(using Context)
+extends TypeMsg(PhantomSymbolNotValueID):
+  protected def msg(using Context): String =
+    i"dummy term capture parameter $sym cannot be used as a value"
+  protected def explain(using Context): String =
+    i"""A term capture parameter is a symbol made up by the compiler to represent a reference
+       |to a real capture parameter in capture sets. For instance, in
+       |
+       |   class A:
+       |     type C^
+       |
+       |there is just a type `A` declared but not a value `A`. Nevertheless, one can write
+       |the selection `(a: A).C` and use a a value, which works because the compiler created a
+       |term capture parameter for `C`. However, these term capture parameters are not real values,
+       |they can only be referred in capture sets."""
+
 class UnusedSymbol(errorText: String, val actions: List[CodeAction] = Nil)(using Context)
 extends Message(UnusedSymbolID):
   def kind = MessageKind.UnusedSymbol
@@ -3518,7 +3534,7 @@ final class OnlyFullyDependentAppliedConstructorType()(using Context)
   override protected def explain(using Context): String = ""
 
 final class IllegalContextBounds(using Context) extends SyntaxMsg(IllegalContextBoundsID):
-  override protected def msg(using Context): String = 
+  override protected def msg(using Context): String =
     i"Context bounds are not allowed in this position"
 
   override protected def explain(using Context): String = ""

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

@@ -89,7 +89,8 @@ class Getters extends MiniPhase with SymTransformer { thisPhase =>
       else d1 = d1.copySymDenotation(initFlags = d1.flags &~ Local)
     d1
   }
-  private val NoGetterNeededFlags = Method | Param | JavaDefined | JavaStatic | CaptureParam
+
+  private val NoGetterNeededFlags = Method | Param | JavaDefined | JavaStatic | PhantomSymbol
 
   val newSetters = util.HashSet[Symbol]()
 

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

@@ -350,7 +350,10 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
       def unusable(msg: Symbol => Message) =
         errorTree(tree, msg(tree.symbol))
       if tree.symbol.is(PhantomSymbol) then
-        unusable(ConstructorProxyNotValue(_))
+        if tree.symbol.isDummyCaptureParam then
+          unusable(DummyCaptureParamNotValue(_))
+        else
+          unusable(ConstructorProxyNotValue(_))
       else if tree.symbol.isContextBoundCompanion then
         unusable(ContextBoundCompanionNotValue(_))
       else
@@ -707,7 +710,6 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         case _ =>
       if parents1 ne info.parents then info.derivedClassInfo(declaredParents = parents1)
       else tp
-    case _ if sym.is(PhantomSymbol) => NoType
     case _ => tp
 
   private def argTypeOfCaseClassThatNeedsAbstractFunction1(sym: Symbol)(using Context): Option[List[Type]] =

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

@@ -434,7 +434,7 @@ object RefChecks {
         }
 
       def emitOverrideError(fullmsg: Message) =
-        if !(hasErrors && member.is(Synthetic) && member.is(Module) || member.is(PhantomSymbol)) then
+        if !(hasErrors && member.is(Synthetic) && member.is(Module) || member.isDummyCaptureParam) then
           // suppress errors relating to synthetic companion objects if other override
           // errors (e.g. relating to the companion class) have already been reported.
           if (member.owner == clazz) report.error(fullmsg, member.srcPos)

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

@@ -649,7 +649,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
      */
     def checkNotShadowed(ownType: Type): Type =
       ownType match
-        case ownType: TermRef if ownType.symbol.is(PhantomSymbol) && !ctx.mode.is(Mode.InCaptureSet) =>
+        case ownType: TermRef
+          if ownType.symbol.is(PhantomSymbol)
+            && !(ctx.mode.is(Mode.InCaptureSet) && ownType.symbol.isDummyCaptureParam) =>
           findRef(name, pt, EmptyFlags, PhantomSymbol, tree.srcPos) match
             case shadowed: TermRef if !shadowed.symbol.maybeOwner.isEmptyPackage =>
               pt match

tests/neg-custom-args/captures/check-dummy-symbol.check

@@ -0,0 +1,12 @@
+-- [E195] Type Error: tests/neg-custom-args/captures/check-dummy-symbol.scala:6:21 -------------------------------------
+6 |def f(a: A): a.C = a.C // error
+  |                   ^^^
+  |                   dummy term capture parameter value C cannot be used as a value
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E195] Type Error: tests/neg-custom-args/captures/check-dummy-symbol.scala:7:15 -------------------------------------
+7 |def g[C^]: C = C // error
+  |               ^
+  |               dummy term capture parameter value C cannot be used as a value
+  |
+  | longer explanation available when compiling with `-explain`