Better error message for polytypes wrapping capturing types

A type like

    [X] -> A ->{c} B

is currently not allowed since it expands to a PolyType wrapping a CapturingType and we
have an implementation restriction that requires PolyTypes to wrap only FunctionTypes. It
would be great if we could lift that implementation restriction. Until we do so, we should
have a better error message, which is commit implements.

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -518,6 +518,22 @@ object Parsers {
         tree
     }
 
+    def makePolyFunction(tparams: List[Tree], body: Tree,
+        kind: String, errorTree: => Tree,
+        start: Offset, arrowOffset: Offset): Tree =
+      atSpan(start, arrowOffset):
+        getFunction(body) match
+          case None =>
+            syntaxError(em"Implementation restriction: polymorphic function ${kind}s must have a value parameter", arrowOffset)
+            errorTree
+          case Some(Function(_, _: CapturesAndResult)) =>
+            // A function tree like this will be desugared
+            // into a capturing type in the typer.
+            syntaxError(em"Implementation restriction: polymorphic function types cannot wrap function types that have capture sets", arrowOffset)
+            errorTree
+          case Some(f) =>
+            PolyFunction(tparams, body)
+
 /* --------------- PLACEHOLDERS ------------------------------------------- */
 
     /** The implicit parameters introduced by `_` in the current expression.
@@ -1539,11 +1555,6 @@ object Parsers {
     private def getFunction(tree: Tree): Option[Function] = tree match {
       case Parens(tree1) => getFunction(tree1)
       case Block(Nil, tree1) => getFunction(tree1)
-      case Function(_, _: CapturesAndResult) =>
-        // A function tree like this will be desugared
-        // into a capturing type in the typer,
-        // so None is returned.
-        None
       case t: Function => Some(t)
       case _ => None
     }
@@ -1757,13 +1768,7 @@ object Parsers {
         else if in.token == ARROW || isPureArrow(nme.PUREARROW) then
           val arrowOffset = in.skipToken()
           val body = toplevelTyp(nestedIntoOK(in.token))
-          atSpan(start, arrowOffset):
-            getFunction(body) match
-              case Some(f) =>
-                PolyFunction(tparams, body)
-              case None =>
-                syntaxError(em"Implementation restriction: polymorphic function types must have a value parameter", arrowOffset)
-                Ident(nme.ERROR.toTypeName)
+          makePolyFunction(tparams, body, "type", Ident(nme.ERROR.toTypeName), start, arrowOffset)
         else
           accept(TLARROW)
           typ()
@@ -2360,14 +2365,7 @@ object Parsers {
           val tparams = typeParamClause(ParamOwner.Type)
           val arrowOffset = accept(ARROW)
           val body = expr(location)
-          atSpan(start, arrowOffset) {
-            getFunction(body) match
-              case Some(f) =>
-                PolyFunction(tparams, f)
-              case None =>
-                syntaxError(em"Implementation restriction: polymorphic function literals must have a value parameter", arrowOffset)
-                errorTermTree(arrowOffset)
-          }
+          makePolyFunction(tparams, body, "literal", errorTermTree(arrowOffset), start, arrowOffset)
         case _ =>
           val saved = placeholderParams
           placeholderParams = Nil

tests/neg-custom-args/captures/polyCaptures.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/captures/polyCaptures.scala:4:22 -------------------------------------------------------
+4 |val runOpsCheck: [C^] -> (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps  // error
+  |                      ^
+  |          Implementation restriction: polymorphic function types cannot wrap function types that have capture sets
+-- Error: tests/neg-custom-args/captures/polyCaptures.scala:5:23 -------------------------------------------------------
+5 |val runOpsCheck2: [C^] => (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error
+  |                       ^
+  |          Implementation restriction: polymorphic function types cannot wrap function types that have capture sets

tests/neg-custom-args/captures/polyCaptures.scala

@@ -0,0 +1,7 @@
+class Box[X](val elem: X)
+
+val runOps = [C^] => (b: Box[() ->{C^} Unit]) => b.elem()
+val runOpsCheck: [C^] -> (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps  // error
+val runOpsCheck2: [C^] => (ops: List[() ->{C^} Unit]) ->{C^} Unit = runOps // error
+
+