Fix #13523: Survive missing Java inner annotation classfiles

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -1127,7 +1127,16 @@ class ClassfileParser(
       val outerName = entry.strippedOuter
       val innerName = entry.originalName
       val owner = classNameToSymbol(outerName)
-      val result = atPhase(typerPhase)(getMember(owner, innerName.toTypeName))
+      val result = owner.denot.infoOrCompleter match
+        case _: StubInfo if hasAnnotation(entry.jflags) =>
+          requiredClass(innerName.toTypeName)
+            // It's okay for the classfiles of Java annotations to be missing
+            // from the classpath. If an annotation is defined as an inner class
+            // we need to avoid forcing the outer class symbol here, and instead
+            // return a new stub symbol for the inner class. This is tested by
+            // `surviveMissingInnerClassAnnot` in AnnotationsTests.scala
+        case _ =>
+          atPhase(typerPhase)(getMember(owner, innerName.toTypeName))
       assert(result ne NoSymbol,
         i"""failure to resolve inner class:
            |externalName = ${entry.externalName},

compiler/test/dotty/tools/AnnotationsTests.scala

@@ -66,3 +66,26 @@ class AnnotationsTest:
           s"A missing annotation while parsing a Java class should be silently ignored but: ${ctx.reporter.summary}")
       }
     }
+
+  @Test def surviveMissingInnerClassAnnot: Unit =
+    withJavaCompiled(
+      VirtualJavaSource("Outer.java",
+        """|package a.b;
+           |public @interface Outer { public @interface Value { @interface Immutable {} } }
+           |""".stripMargin),
+      VirtualJavaSource("Baz.java",
+        """|package a.b;
+           |@Outer.Value.Immutable abstract class Baz {}""".stripMargin)
+    ) { javaOutputDir =>
+      Files.delete(javaOutputDir.resolve("a/b/Outer.class"))
+      Files.delete(javaOutputDir.resolve("a/b/Outer$Value.class"))
+      Files.delete(javaOutputDir.resolve("a/b/Outer$Value$Immutable.class"))
+      inCompilerContext(javaOutputDir.toString + File.pathSeparator + TestConfiguration.basicClasspath) {
+        val cls = requiredClass("a.b.Baz")
+        val annots = cls.annotations.map(_.tree)
+        assert(annots == Nil,
+          s"class Baz should have no visible annotations since Outer.Value.Immutable is not on the classpath, but found: $annots")
+        assert(!ctx.reporter.hasErrors && !ctx.reporter.hasWarnings,
+          s"A missing annotation while parsing a Java class should be silently ignored but: ${ctx.reporter.summary}")
+      }
+    }