Avoid premature widening of type in recheckSelection

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -288,7 +288,7 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  outcome of a `mightSubcapture` test. It picks `{f}` if this might subcapture Cr
      *  and Cr otherwise.
      */
-    override def recheckSelection(tree: Select, qualType: Type, name: Name)(using Context) = {
+    override def recheckSelection(tree: Select, qualType: Type, name: Name, pt: Type)(using Context) = {
       def disambiguate(denot: Denotation): Denotation = denot match
         case MultiDenotation(denot1, denot2) =>
           // This case can arise when we try to merge multiple types that have different
@@ -310,8 +310,12 @@ class CheckCaptures extends Recheck, SymTransformer:
       else
         val qualCs = qualType.captureSet
         capt.println(i"intersect $qualType, ${selType.widen}, $qualCs, $selCs in $tree")
-        if qualCs.mightSubcapture(selCs) then
+        if qualCs.mightSubcapture(selCs)
+            && !selCs.mightSubcapture(qualCs)
+            && !pt.stripCapturing.isInstanceOf[SingletonType]
+        then
           selType.widen.stripCapturing.capturing(qualCs)
+            .showing(i"alternate type for select $tree: $selType --> $result, $qualCs / $selCs", capt)
         else
           selType
     }//.showing(i"recheck sel $tree, $qualType = $result")

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

@@ -21,6 +21,7 @@ import util.Property
 import StdNames.nme
 import reporting.trace
 import annotation.constructorOnly
+import cc.CaptureSet.IdempotentCaptRefMap
 
 object Recheck:
   import tpd.*
@@ -144,9 +145,9 @@ abstract class Recheck extends Phase, SymTransformer:
     def recheckIdent(tree: Ident)(using Context): Type =
       tree.tpe
 
-    def recheckSelect(tree: Select)(using Context): Type =
+    def recheckSelect(tree: Select, pt: Type)(using Context): Type =
       val Select(qual, name) = tree
-      recheckSelection(tree, recheck(qual).widenIfUnstable, name)
+      recheckSelection(tree, recheck(qual).widenIfUnstable, name, pt)
 
     def recheckSelection(tree: Select, qualType: Type, name: Name,
         sharpen: Denotation => Denotation)(using Context): Type =
@@ -162,8 +163,8 @@ abstract class Recheck extends Phase, SymTransformer:
 
 
     /** Keep the symbol of the `select` but re-infer its type */
-    def recheckSelection(tree: Select, qualType: Type, name: Name)(using Context): Type =
-      recheckSelection(tree, qualType, name, sharpen = identity)
+    def recheckSelection(tree: Select, qualType: Type, name: Name, pt: Type)(using Context): Type =
+      recheckSelection(tree, qualType, name, sharpen = identity[Denotation])
 
     def recheckBind(tree: Bind, pt: Type)(using Context): Type = tree match
       case Bind(name, body) =>
@@ -200,7 +201,7 @@ abstract class Recheck extends Phase, SymTransformer:
      *  to FromJavaObject since it got lost in ElimRepeated
      */
     private def mapJavaArgs(formals: List[Type])(using Context): List[Type] =
-      val tm = new TypeMap:
+      val tm = new TypeMap with IdempotentCaptRefMap:
         def apply(t: Type) = t match
           case t: TypeRef if t.symbol == defn.ObjectClass => defn.FromJavaObjectType
           case _ => mapOver(t)
@@ -357,7 +358,7 @@ abstract class Recheck extends Phase, SymTransformer:
         val sym = tree.symbol
         tree match
           case tree: Ident => recheckIdent(tree)
-          case tree: Select => recheckSelect(tree)
+          case tree: Select => recheckSelect(tree, pt)
           case tree: Bind => recheckBind(tree, pt)
           case tree: ValOrDefDef =>
             if tree.isEmpty then NoType

tests/pos-custom-args/captures/enum-extends.scala

@@ -0,0 +1,4 @@
+enum E:
+
+  case A extends E
+