src/jdk.compiler/share/classes/com/sun/tools/javac/platform/JDKPlatformProvider.java

*** 88,97 ****
--- 88,99 ----
                      }
                  }
              } catch (IOException | ProviderNotFoundException ex) {
              }
          }
+         // Workaround until full support for --release 9 distinct from --release 10
+         SUPPORTED_JAVA_PLATFORM_VERSIONS.add(targetNumericVersion(Target.JDK1_9));
          SUPPORTED_JAVA_PLATFORM_VERSIONS.add(targetNumericVersion(Target.DEFAULT));
      }
  
      private static String targetNumericVersion(Target target) {
          return Integer.toString(target.ordinal() - Target.JDK1_1.ordinal() + 1);
*** 106,116 ****
              this.version = version;
          }
  
          @Override
          public Collection<Path> getPlatformPath() {
!             if (Target.lookup(version) == Target.DEFAULT) {
                  return null;
              }
  
              List<Path> paths = new ArrayList<>();
              Path file = findCtSym();
--- 108,120 ----
              this.version = version;
          }
  
          @Override
          public Collection<Path> getPlatformPath() {
!             // Comparison should be == Target.DEFAULT once --release 9
!             // is distinct from 10
!             if (Target.lookup(version).compareTo(Target.JDK1_9)  >=  0) {
                  return null;
              }
  
              List<Path> paths = new ArrayList<>();
              Path file = findCtSym();