javac 9 fails to report an error for a missing import in the following demo. The missing import is unused, and another class on the classpath contains a reference to the missing symbol. The issue only reproduces with annotation processing enabled.

This affects 9-ea+177 but not 1.8.0_152-ea-b04 and earlier versions.

Repro:

=== ./P.java
import static java.nio.charset.StandardCharsets.UTF_8;

import java.io.IOError;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.element.TypeElement;

@SupportedAnnotationTypes("*")
public class P extends AbstractProcessor {
  boolean first = true;
  @Override
  public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    if (first) {
      try (OutputStream os =
          processingEnv.getFiler().createSourceFile("no.such.Hello").openOutputStream()) {
        os.write("package no.such; public class Hello {}".getBytes(UTF_8));
      } catch (IOException e) {
        throw new IOError(e);
      }
      first = false;
    }
    return false;
  }
}
=== ./no/such/Bar.java
package no.such;
public class Bar {
  Foo.Inner i;
}
=== ./no/such/Foo.java
package no.such;
public class Foo {
  public static class Inner {}
}
=== ./no/such/T.java
package no.such;
import no.such.Foo.Inner;
@Deprecated
class T {
  Bar b;
  Hello h;
}

$ javac P.java no/such/Bar.java no/such/Foo.java
$ rm no/such/Foo*
$ javac -processor P no/such/T.java

The compilation succeeds unexpectedly. T.java references no.such.Foo, which was deleted and is not present on the classpath or sourcepath. Performing the same compilation except compiling the generated source explicitly fails with the expected error:

$ javac -sourcepath : -cp lib.jar no/such/T.java no/such/Hello.java
no/such/T.java:2: error: package no.such.Foo does not exist
import no.such.Foo.Inner;
                  ^
1 error

I suspect that the error is being deferred when it is first detected since the missing symbol could be generated during a subsequent annotation processing round, and javac is failing to detect the error during later rounds.