Add back isSpecializableTuple exclusion

... and leave a comment now that I understand it.

Author: dwijnand

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

@@ -1635,6 +1635,7 @@ class Definitions {
       case List(x)    => Tuple1SpecializedParamClasses().contains(x.classSymbol)
       case List(x, y) => Tuple2SpecializedParamClasses().contains(x.classSymbol) && Tuple2SpecializedParamClasses().contains(y.classSymbol)
       case _          => false
+    && base.owner.denot.info.member(base.name.specializedName(args)).exists // when dotc compiles the stdlib there are no specialised classes
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(using Context): Boolean =
     paramTypes.length <= 2