Add DocArtifact flag

This flag is used to identify symbols that are introduced while cooking
the comments (typically, these are the symbols created by the `@usecase`
annotation). The flag is used to identify those symbols in the
TreeChecker.

Author: Duhemm

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

@@ -432,6 +432,9 @@ object Flags {
   /** A denotation that is valid in all run-ids */
   final val Permanent = commonFlag(62, "<permanent>")
 
+  /** A symbol introduced while cooking comments */
+  final val DocArtifact = commonFlag(63, "<doc-artifact>")
+
 // --------- Combined Flag Sets and Conjunctions ----------------------
 
   /** Flags representing source modifiers */

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -377,6 +377,7 @@ class TreeChecker extends Phase with SymTransformer {
 
       def isNonMagicalMethod(x: Symbol) =
         x.is(Method) &&
+          !x.is(DocArtifact) &&
           !x.isCompanionMethod &&
           !x.isValueClassConvertMethod &&
           !(x.is(Macro) && ctx.phase.refChecked)

compiler/src/dotty/tools/dotc/typer/Docstrings.scala

@@ -23,7 +23,8 @@ trait Docstrings { self: Typer =>
         val usecases = docbase.docstring(sym).map(_.usecases).getOrElse(Nil)
 
         usecases.foreach { usecase =>
-          enterSymbol(createSymbol(usecase.untpdCode))
+          val createdSymbol = enterSymbol(createSymbol(usecase.untpdCode))
+          createdSymbol.setFlag(Flags.DocArtifact)
 
           typedStats(usecase.untpdCode :: Nil, owner) match {
             case List(df: tpd.DefDef) => usecase.tpdCode = df