Fix #8150: Improve screening of legal refinements

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

@@ -134,7 +134,7 @@ object Flags {
       }
 
     /** The list of non-empty names of flags that are set in the given flag set */
-    def flagStrings(privateWithin: String): Seq[String] = {
+    def flagStrings(privateWithin: String = ""): Seq[String] = {
       var rawStrings = (2 to MaxFlag).flatMap(x.flagString(_)) // DOTTY problem: cannot drop with (_)
       if (!privateWithin.isEmpty && !x.is(Protected))
       	rawStrings = rawStrings :+ "private"

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

@@ -3844,19 +3844,21 @@ object Parsers {
      */
     def refineStatSeq(): List[Tree] = {
       val stats = new ListBuffer[Tree]
-      def checkLegal(tree: Tree): List[Tree] = {
-        val isLegal = tree match {
-          case tree: ValDef => tree.rhs.isEmpty && !tree.mods.flags.is(Mutable)
-          case tree: DefDef => tree.rhs.isEmpty
-          case tree: TypeDef => true
-          case _ => false
-        }
-        if (isLegal) tree :: Nil
-        else {
-          syntaxError("illegal refinement", tree.span)
-          Nil
-        }
-      }
+      def checkLegal(tree: Tree): List[Tree] =
+        val problem = tree match
+          case tree: MemberDef if !(tree.mods.flags & ModifierFlags).isEmpty =>
+            i"refinement cannot be ${(tree.mods.flags & ModifierFlags).flagStrings().mkString("`", "`, `", "`")}"
+          case tree: ValOrDefDef =>
+            if tree.rhs.isEmpty then ""
+            else "refinement in cannot have a right-hand side"
+          case tree: TypeDef =>
+            if !tree.isClassDef then ""
+            else "refinement cannot be a class or trait"
+          case _ =>
+            "this kind of definition cannot be a refinement"
+        if problem.isEmpty then tree :: Nil
+        else { syntaxError(problem, tree.span); Nil }
+
       while (!isStatSeqEnd) {
         if (isDclIntro)
           stats ++= checkLegal(defOrDcl(in.offset, Modifiers()))

tests/neg/i8150.scala

@@ -0,0 +1,3 @@
+trait A
+trait B
+type T = {given(given a: A) as B} // error: refinement cannot be `given`