Ignore best effort settings in repl

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -87,8 +87,21 @@ class ReplDriver(settings: Array[String],
     setupRootCtx(this.settings ++ settings, rootCtx)
   }
 
+  private val incompatibleOptions: Seq[String] = Seq(
+    initCtx.settings.YbestEffort.name,
+    initCtx.settings.YwithBestEffortTasty.name
+  )
+
   private def setupRootCtx(settings: Array[String], rootCtx: Context) = {
-    setup(settings, rootCtx) match
+    val incompatible = settings.intersect(incompatibleOptions)
+    val filteredSettings =
+      if !incompatible.isEmpty then
+        inContext(rootCtx) {
+          out.println(i"Options incompatible with repl will be ignored: ${incompatible.mkString(", ")}")
+        }
+        settings.filter(!incompatible.contains(_))
+      else settings
+    setup(filteredSettings, rootCtx) match
       case Some((files, ictx)) => inContext(ictx) {
         shouldStart = true
         if files.nonEmpty then out.println(i"Ignoring spurious arguments: $files%, %")

compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -456,6 +456,17 @@ class ReplCompilerTests extends ReplTest:
       assertTrue(last, last.startsWith("val res0: tpolecat.type = null"))
       assertTrue(last, last.endsWith("""// result of "res0.toString" is null"""))
 
+  @Test def `i21431 filter out best effort options`: Unit =
+    initially:
+      run(":settings -Ybest-effort -Ywith-best-effort-tasty")
+    .andThen:
+      run("0") // check for crash
+      val last = lines()
+      println(last)
+      assertTrue(last(0), last(0) == ("Options incompatible with repl will be ignored: -Ybest-effort, -Ywith-best-effort-tasty"))
+      assertTrue(last(1), last(1) == ("val res0: Int = 0"))
+
+
 object ReplCompilerTests:
 
   private val pattern = Pattern.compile("\\r[\\n]?|\\n");