Tell user to blaze clean --expunge.

PiperOrigin-RevId: 203813666

src/main/java/com/google/devtools/build/lib/rules/apple/XcodeConfig.java

diff --git a/src/main/java/com/google/devtools/build/lib/rules/apple/XcodeConfig.java b/src/main/java/com/google/devtools/build/lib/rules/apple/XcodeConfig.java
index d9ae4a0..943d751 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/apple/XcodeConfig.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/apple/XcodeConfig.java
@@ -178,10 +178,10 @@
         return explicitVersion;
       } else {
         throw new XcodeConfigException(String.format(
-            "--xcode_version=%s specified, but '%s' is not an available Xcode version. "
-            + "available versions: [%s]",
-            versionOverrideFlag, versionOverrideFlag,
-            printableXcodeVersions(xcodeVersionRules)));
+            "--xcode_version=%1$s specified, but '%1$s' is not an available Xcode version. "
+            + "available versions: [%2$s]. If you believe you have '%1$s' installed, try running "
+            + "\"blaze clean --expunge\", and then re-run your command.",
+            versionOverrideFlag, printableXcodeVersions(xcodeVersionRules)));
       }
     }
 

src/test/java/com/google/devtools/build/lib/rules/apple/XcodeConfigTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/rules/apple/XcodeConfigTest.java b/src/test/java/com/google/devtools/build/lib/rules/apple/XcodeConfigTest.java
index 5ef83c8..161164f 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/apple/XcodeConfigTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/apple/XcodeConfigTest.java
@@ -303,7 +303,7 @@
     reporter.removeHandler(failFastHandler);
     getConfiguredTarget("//xcode:foo");
     assertContainsEvent("--xcode_version=6 specified, but '6' is not an available Xcode version. "
-        + "available versions: [5.1.2, 8.4]");
+        + "available versions: [5.1.2, 8.4]. If you believe you have '6' installed");
   }
 
   @Test