Handle TypeError during Erasure phase

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

@@ -20,6 +20,7 @@ import core.Decorators.*
 import core.Constants.*
 import core.Definitions.*
 import core.Annotations.BodyAnnotation
+import core.TypeError
 import typer.NoChecking
 import inlines.Inlines
 import typer.ProtoTypes.*
@@ -49,7 +50,7 @@ class Erasure extends Phase with DenotTransformer {
   override def changesMembers: Boolean = true // the phase adds bridges
   override def changesParents: Boolean = true // the phase drops Any
 
-  def transform(ref: SingleDenotation)(using Context): SingleDenotation = ref match {
+  def transform(ref: SingleDenotation)(using Context): SingleDenotation = try ref match {
     case ref: SymDenotation =>
       def isCompacted(symd: SymDenotation) =
         symd.isAnonymousFunction && {
@@ -130,7 +131,10 @@ class Erasure extends Phase with DenotTransformer {
         ref.symbol, transformInfo(ref.symbol, ref.symbol.info), currentStablePeriod, ref.prefix)
     case _ =>
       ref.derivedSingleDenotation(ref.symbol, transformInfo(ref.symbol, ref.symbol.info))
-  }
+  } catch case ex: TypeError =>
+    // Handle missing types from dependencies (e.g., JDK version mismatch)
+    report.error(ex.toMessage, ref.symbol.srcPos)
+    ref  // Keep old denotation on error
 
   private val eraser = new Erasure.Typer(this)
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -149,7 +149,7 @@ class CompilationTests {
 
   @Test def negAll: Unit = {
     implicit val testGroup: TestGroup = TestGroup("compileNeg")
-    aggregateTests(
+    var tests = List(
       compileFilesInDir("tests/neg", defaultOptions, FileFilter.exclude(TestSources.negScala2LibraryTastyExcludelisted)),
       compileFilesInDir("tests/neg-deep-subtype", allowDeepSubtypes),
       compileFilesInDir("tests/neg-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking", "-language:experimental.separationChecking", "-source", "3.8")),
@@ -160,7 +160,13 @@ class CompilationTests {
         "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
         defaultOptions),
       compileFile("tests/neg/i7575.scala", defaultOptions.withoutLanguageFeatures),
-    ).checkExpectedErrors()
+    )
+
+    // i20491: Test JDK version mismatch error - only run on JDK < 21
+    if !scala.util.Properties.isJavaAtLeast("21") then
+      tests ::= compileFile("tests/neg-custom-args/i20491/Test.scala", defaultOptions.withClasspath("tests/neg-custom-args/i20491/cp"))
+
+    aggregateTests(tests*).checkExpectedErrors()
   }
 
   @Test def fuzzyAll: Unit = {

tests/neg-custom-args/i20491/README.md

@@ -0,0 +1,16 @@
+# Test case for https://github.com/scala/scala3/issues/20491
+
+The `cp/ox/Ox.class` is compiled from the following source using JDK 21:
+
+```scala
+// Ox.scala
+//> using jvm 21
+package ox
+import java.util.concurrent.StructuredTaskScope
+trait Ox:
+  def scope: StructuredTaskScope[Any]
+```
+
+`$ scala-cli compile Ox.scala -d ./cp`
+
+This test must be run on JDK < 21 to trigger the missing type error for `java.util.concurrent.StructuredTaskScope`.

tests/neg-custom-args/i20491/Test.check

@@ -0,0 +1,3 @@
+-- Error: Ox.scala:6:6 -------------------------------------------------------------------------------------------------
+Cannot resolve reference to type java.util.concurrent.type.StructuredTaskScope.
+The classfile defining the type might be missing from the classpath.

tests/neg-custom-args/i20491/Test.scala

@@ -0,0 +1,2 @@
+// anypos-error
+given ox.Ox = ???