Don't add capture sets to types that cannot capture anything

Author: odersky

compiler/src/dotty/tools/dotc/CaptureOps.scala

@@ -57,7 +57,9 @@ object CaptureOps:
       getBoxed(tp, enabled = false)
 
     /** If this type appears as an expected type of a term, does it imply
-     *  that the term should be boxed? ^^^ Special treat Any?
+     *  that the term should be boxed?
+     *  ^^^ Special treat Any? - but the current status is more conservative in that
+     *  it counts free variables in expressions that have Any as expected type.
      */
     def needsBox(using Context): Boolean = tp match
       case _: TypeVar => true
@@ -73,25 +75,15 @@ object CaptureOps:
       case _ => false
 
     def canHaveInferredCapture(using Context): Boolean = tp match
-      case tp: CapturingType =>
-        false
-      case tp: TypeRef =>
-        if tp.symbol.isClass then
-          !tp.symbol.isValueClass && tp.symbol != defn.AnyClass
-        else
-          tp.underlying.canHaveInferredCapture
+      case tp: TypeRef if tp.symbol.isClass =>
+        !tp.symbol.isValueClass && tp.symbol != defn.AnyClass
       case tp: TypeProxy =>
-        tp.underlying.canHaveInferredCapture
+        tp.superType.canHaveInferredCapture
       case tp: AndType =>
         tp.tp1.canHaveInferredCapture && tp.tp2.canHaveInferredCapture
       case tp: OrType =>
         tp.tp1.canHaveInferredCapture || tp.tp2.canHaveInferredCapture
       case _ =>
         false
 
-    def addCaptureVars(using Context): Type =
-      if ctx.settings.Ycc.value && canHaveInferredCapture then
-        CapturingType(tp, CaptureSet.Var()) // ^^^ go deep
-      else
-        tp
 end CaptureOps

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

@@ -72,12 +72,14 @@ class CheckCaptures extends Recheck:
             case _ =>
               mapOver(t)
         def addVars(tp: Type): Type =
-          val tp1 = tp match
-            case tp @ AppliedType(tycon, args) =>
-              tp.derivedAppliedType(tycon, args.map(addVars))
-            case _ => // ^^^ go deeper in other types as well
-              tp
-          CapturingType(tp1, CaptureSet.Var())
+          if tp.canHaveInferredCapture then
+            val tp1 = tp match
+              case tp @ AppliedType(tycon, args) =>
+                tp.derivedAppliedType(tycon, args.map(addVars))
+              case _ => // ^^^ go deeper in other types as well
+                tp
+            CapturingType(tp1, CaptureSet.Var())
+          else tp
         tp match
           case tp: MethodOrPoly =>
             tp.derivedLambdaType(resType = reinfer(tp.resType))