diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 1aabb85f5919..0ac5eaefce20 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1879,6 +1879,13 @@ object SymDenotations {
     private var baseDataCache: BaseData = BaseData.None
     private var memberNamesCache: MemberNames = MemberNames.None
 
+    /** Set of parent types for which a `BadSymbolicReference` has already
+     *  been reported by `computeBaseData`. Used so the same diagnostic is
+     *  not emitted multiple times when `baseData` is recomputed (e.g. from
+     *  a separate `derivesFrom` invocation). See scala/scala3#20010.
+     */
+    private var reportedMissingParents: Set[Type] = Set.empty
+
     private def memberCache(using Context): EqHashMap[Name, PreDenotation] = {
       if (myMemberCachePeriod != ctx.period) {
         myMemberCache = EqHashMap()
@@ -2061,7 +2068,29 @@ object SymDenotations {
         case p :: parents1 =>
           p.classSymbol match {
             case pcls: ClassSymbol => builder.addAll(pcls.baseClasses)
-            case _ => assert(isRefinementClass || p.isError || ctx.mode.is(Mode.Interactive) || ctx.tolerateErrorsForBestEffort, s"$this has non-class parent: $p")
+            case _ =>
+              // The parent type couldn't be resolved to a class, e.g.
+              // because a transitive dependency was removed from the
+              // classpath. Report a BadSymbolicReference-style error
+              // rather than crashing with an internal assertion. See
+              // scala/scala3#20010.
+              if p.typeSymbol == NoSymbol && !isRefinementClass && !p.isError
+                 && !ctx.mode.is(Mode.Interactive) && !ctx.tolerateErrorsForBestEffort
+              then
+                if !reportedMissingParents.contains(p) then
+                  reportedMissingParents = reportedMissingParents + p
+                  val file = symbol.associatedFile
+                  val (location, src) =
+                    if file != null then (i" in $file", file.toString)
+                    else ("", "the signature")
+                  report.error(
+                    em"""Bad symbolic reference. A signature$location
+                        |refers to ${p.show} as a parent of ${symbol.showLocated}, but it is not available.
+                        |It may be completely missing from the current classpath, or the version on
+                        |the classpath might be incompatible with the version used when compiling $src.""",
+                    symbol.srcPos)
+              else
+                assert(isRefinementClass || p.isError || ctx.mode.is(Mode.Interactive) || ctx.tolerateErrorsForBestEffort, s"$this has non-class parent: $p")
           }
           traverse(parents1)
         case nil =>
@@ -2419,6 +2448,11 @@ object SymDenotations {
             case pcls: ClassSymbol =>
               for name <- pcls.memberNames(keepOnly) do
                 maybeAdd(name)
+            case _ =>
+              // Parent failed to resolve to a class (the missing
+              // reference has been reported by computeBaseData).
+              // Skip here to avoid a secondary MatchError.
+              // See scala/scala3#20010.
         val ownSyms =
           if (keepOnly eq implicitFilter)
             if (this.is(Package)) Iterator.empty
diff --git a/sbt-test/tasty-compat/i20010/Repro.scala b/sbt-test/tasty-compat/i20010/Repro.scala
new file mode 100644
index 000000000000..392cd009482b
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/Repro.scala
@@ -0,0 +1 @@
+class MyTest extends child.ValidatingTest
diff --git a/sbt-test/tasty-compat/i20010/a/ParsingTest.scala b/sbt-test/tasty-compat/i20010/a/ParsingTest.scala
new file mode 100644
index 000000000000..ebddc4634b99
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/a/ParsingTest.scala
@@ -0,0 +1,3 @@
+package parent
+
+trait ParsingTest
diff --git a/sbt-test/tasty-compat/i20010/b/ValidatingTest.scala b/sbt-test/tasty-compat/i20010/b/ValidatingTest.scala
new file mode 100644
index 000000000000..7bf487d1e313
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/b/ValidatingTest.scala
@@ -0,0 +1,3 @@
+package child
+
+abstract class ValidatingTest extends parent.ParsingTest
diff --git a/sbt-test/tasty-compat/i20010/build.sbt b/sbt-test/tasty-compat/i20010/build.sbt
new file mode 100644
index 000000000000..d4299b04dbf4
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/build.sbt
@@ -0,0 +1,56 @@
+import sbt.internal.util.ConsoleAppender
+
+// Reproduces https://github.com/scala/scala3/issues/20010
+//
+// Three modules:
+//   a  : defines `parent.ParsingTest`
+//   b  : defines `child.ValidatingTest extends parent.ParsingTest`,
+//        compiled with `a` on the classpath; its output ends up in `c-input`
+//   c  : root project that extends `child.ValidatingTest`, but only sees
+//        `c-input` on its classpath – `a`'s outputs are deliberately not
+//        exposed, so loading `ValidatingTest`'s TASTy can no longer resolve
+//        its `parent.ParsingTest` parent.
+//
+// Before the fix the compiler crashed with
+//   `java.lang.AssertionError: class ValidatingTest has non-class parent: ...`
+// Now it must emit a clean `Bad symbolic reference` error.
+
+lazy val assertCleanMissingRefError = taskKey[Unit](
+  "checks that compiling c reports `Bad symbolic reference` rather than crashing"
+)
+lazy val resetMessages = taskKey[Unit]("empties the messages list")
+
+lazy val a = project.in(file("a"))
+  .settings(
+    Compile / classDirectory := (ThisBuild / baseDirectory).value / "a-only"
+  )
+
+lazy val b = project.in(file("b"))
+  .settings(
+    Compile / unmanagedClasspath += (ThisBuild / baseDirectory).value / "a-only",
+    Compile / classDirectory := (ThisBuild / baseDirectory).value / "c-input"
+  )
+
+lazy val c = project.in(file("."))
+  .settings(
+    // Only `b`'s outputs are visible here – `a-only` is *not* on the classpath.
+    Compile / unmanagedClasspath += (ThisBuild / baseDirectory).value / "c-input",
+    Compile / classDirectory := (ThisBuild / baseDirectory).value / "c-output",
+    extraAppenders := { _ => Seq(ConsoleAppender(FakePrintWriter)) },
+    resetMessages := { FakePrintWriter.resetMessages },
+    assertCleanMissingRefError := {
+      val msgs = FakePrintWriter.messages
+      assert(
+        msgs.exists(_.contains("Bad symbolic reference")),
+        s"expected 'Bad symbolic reference' in compiler output, got: ${msgs.mkString("\n")}"
+      )
+      assert(
+        !msgs.exists(_.contains("non-class parent")),
+        s"compiler crashed with 'non-class parent' assertion; got: ${msgs.mkString("\n")}"
+      )
+      assert(
+        !msgs.exists(_.contains("java.lang.AssertionError")),
+        s"compiler crashed with AssertionError; got: ${msgs.mkString("\n")}"
+      )
+    }
+  )
diff --git a/sbt-test/tasty-compat/i20010/project/DottyInjectedPlugin.scala b/sbt-test/tasty-compat/i20010/project/DottyInjectedPlugin.scala
new file mode 100644
index 000000000000..fb946c4b8c61
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/project/DottyInjectedPlugin.scala
@@ -0,0 +1,11 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion")
+  )
+}
diff --git a/sbt-test/tasty-compat/i20010/project/FakePrintWriter.scala b/sbt-test/tasty-compat/i20010/project/FakePrintWriter.scala
new file mode 100644
index 000000000000..14d3bcdba5b4
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/project/FakePrintWriter.scala
@@ -0,0 +1,6 @@
+object FakePrintWriter extends java.io.PrintWriter("fake-print-writer") {
+  @volatile var messages = List.empty[String]
+  def resetMessages = messages = List.empty[String]
+  override def println(x: String): Unit = messages = x :: messages
+  override def print(x: String): Unit = messages = x :: messages
+}
diff --git a/sbt-test/tasty-compat/i20010/test b/sbt-test/tasty-compat/i20010/test
new file mode 100644
index 000000000000..9d72fe641dd0
--- /dev/null
+++ b/sbt-test/tasty-compat/i20010/test
@@ -0,0 +1,10 @@
+# compile library a (defines parent.ParsingTest)
+> a/compile
+# compile library b (defines child.ValidatingTest extends parent.ParsingTest)
+> b/compile
+# compile c without a on the classpath.
+# This used to crash the compiler with an internal `non-class parent` assertion.
+# After the fix it must fail with a clean `Bad symbolic reference` error.
+> resetMessages
+-> c/compile
+> assertCleanMissingRefError