Better error message for "annot have an inferred type"

Author: odersky

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

@@ -1120,18 +1120,23 @@ class CheckCaptures extends Recheck, SymTransformer:
             }
             if !canUseInferred then
               val inferred = t.tpt.knownType
-              def checkPure(tp: Type) = tp match
-                case CapturingType(_, refs)
-                if !refs.elems.filter(isNotPureThis).isEmpty =>
-                  val resultStr = if t.isInstanceOf[DefDef] then " result" else ""
-                  report.error(
-                    em"""Non-local $sym cannot have an inferred$resultStr type
-                        |$inferred
-                        |with non-empty capture set $refs.
-                        |The type needs to be declared explicitly.""".withoutDisambiguation(),
-                    t.srcPos)
-                case _ =>
-              inferred.foreachPart(checkPure, StopAt.Static)
+              val checkPure = new TypeTraverser:
+                def traverse(tp: Type) = tp match
+                  case CapturingType(_, refs)
+                  if !refs.elems.filter(isNotPureThis).isEmpty =>// && variance >= 0 =>
+                    val resultStr = if t.isInstanceOf[DefDef] then " result" else ""
+                    val partStr =
+                      if tp eq inferred then "with non-empty capture set"
+                      else i"since its part $tp has non-empty capture set"
+                    report.error(
+                      em"""Non-local $sym cannot have an inferred$resultStr type
+                          |$inferred
+                          |$partStr $refs.
+                          |The type needs to be declared explicitly.""".withoutDisambiguation(),
+                      t.srcPos)
+                  case _ =>
+                    traverseChildren(tp)
+              checkPure.traverse(inferred)
           case t @ TypeApply(fun, args) =>
             fun.knownType.widen match
               case tl: PolyType =>