Add addition operation for snippet compiler arg

Author: pikinier20

scaladoc/src/dotty/tools/scaladoc/renderers/UncycloDocRenderer.scala

@@ -13,8 +13,15 @@ class DocRender(signatureRenderer: SignatureRenderer, snippetChecker: SnippetChe
   private val snippetCheckingFuncFromMember: Member => SnippetChecker.SnippetCheckingFunc =
     (m: Member) => {
       (str: String, lineOffset: SnippetChecker.LineOffset, argOverride: Option[SnippetCompilerArg]) => {
-          val arg = argOverride.getOrElse(
-            ctx.snippetCompilerArgs.get(m).getOrElse(SnippetCompilerArg.default)
+          val pathBasedArg = ctx.snippetCompilerArgs.get(m).fold(
+            SnippetCompilerArg.default
+          )(
+            _ + SnippetCompilerArg.default
+          )
+          val arg = argOverride.fold(
+            pathBasedArg
+          )(
+            _ + pathBasedArg
           )
 
           snippetChecker.checkSnippet(str, m.docs.map(_.snippetCompilerData), arg, lineOffset).foreach { _ match {

scaladoc/src/dotty/tools/scaladoc/snippets/SnippetCompilerArgs.scala

@@ -3,6 +3,11 @@ package snippets
 
 case class SnippetCompilerArg(flags: Set[SCFlags]):
   def is(flag: SCFlags): Boolean = flags.contains(flag)
+  def +(other: SnippetCompilerArg): SnippetCompilerArg = {
+    val allNoncompatbileFlags = flags.flatMap(_.forbiddenFlags)
+    val compatibleFlags = other.flags.filter(flag => !allNoncompatbileFlags.contains(flag))
+    copy(flags = flags ++ compatibleFlags)
+  }
 
 object SnippetCompilerArg:
   def default: SnippetCompilerArg = SnippetCompilerArg(