Add check for parents in Quotes (#i19842)

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -253,6 +253,7 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
         (cdef.name.toString, cdef.constructor, cdef.parents, cdef.self, rhs.body)
 
       def module(module: Symbol, parents: List[Tree /* Term | TypeTree */], body: List[Statement]): (ValDef, ClassDef) = {
+        xCheckMacroAssert(parents.nonEmpty && !parents.head.tpe.dealias.classSymbol.is(dotc.core.Flags.Trait), "First parent must be a class")
         val cls = module.moduleClass
         val clsDef = ClassDef(cls, parents, body)
         val newCls = Apply(Select(New(TypeIdent(cls)), cls.primaryConstructor), Nil)

tests/neg-macros/i19842.check

@@ -1,28 +1,10 @@
 -- Error: tests/neg-macros/i19842/Test.scala:9:50 ----------------------------------------------------------------------
 9 |@main def Test = summon[Serializer[ValidationCls]] // error
   |                                                  ^
-  |Malformed tree was found while expanding macro with -Xcheck-macros.
-  |The tree does not conform to the compiler's tree invariants.
+  |                                                  Exception occurred while executing macro expansion.
+  |                                                  java.lang.AssertionError: First parent must be a class
+  |                                                  	at Macros$.makeSerializer(Macro.scala:24)
   |
-  |Macro was:
-  |scala.quoted.runtime.Expr.splice[Serializer[ValidationCls]](((contextual$2: scala.quoted.Quotes) ?=> Macros.makeSerializer[ValidationCls](scala.quoted.Type.of[ValidationCls](contextual$2), contextual$2)))
-  |
-  |The macro returned:
-  |{
-  |  object objectSerializer$macro$1 extends Serializer[ValidationCls] { this: objectSerializer$macro$1.type =>
-  |    
-  |  }
-  |  objectSerializer$macro$1
-  |}
-  |
-  |Error:
-  |assertion failed: Parents of class symbol differs from the parents in the tree for object objectSerializer$macro$1
-  |
-  |Parents in symbol: [class Object, trait Serializer]
-  |Parents in tree: [trait Serializer]
-  |
-  |
-  |stacktrace available when compiling with `-Ydebug`
   |---------------------------------------------------------------------------------------------------------------------
   |Inline stack trace
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

tests/neg-macros/i19842/Macro.scala

@@ -15,7 +15,6 @@ object Macros {
       name,
       Flags.Implicit,
       Flags.EmptyFlags,
-      // Without TypeRep.of[Object] it would fail with java.lang.AssertionError: assertion failed: First parent must be a class
       List(TypeRepr.of[Object], TypeRepr.of[Serializer[T]]),
       _ => Nil,
       Symbol.noSymbol