Clarify sanity check error message

RELNOTES: None
PiperOrigin-RevId: 173302354

2 files changed, 7 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index 8dfe097abf..c8377d69fc 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -186,7 +186,7 @@ public final class EvalUtils {
       return parent;
     }
     Preconditions.checkArgument(SkylarkValue.class.isAssignableFrom(c),
-        "%s is not allowed as a Skylark value", c);
+        "%s is not allowed as a Skylark value (getSkylarkType() failed)", c);
     return c;
   }
 
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
index 6d8c1a8667..b47521d31e 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
@@ -602,9 +602,14 @@ public abstract class SkylarkType implements Serializable {
    * Throws EvalException if the type of the object is not allowed to be present in Skylark.
    */
   static void checkTypeAllowedInSkylark(Object object, Location loc) throws EvalException {
+    // TODO(bazel-team): Unify this check with the logic in EvalUtils.getSkylarkType(). Might
+    // break some providers whose contents don't implement SkylarkValue, aren't wrapped in
+    // SkylarkList, etc.
     if (!isTypeAllowedInSkylark(object)) {
       throw new EvalException(
-          loc, "internal error: type '" + object.getClass().getSimpleName() + "' is not allowed");
+          loc,
+          "internal error: type '" + object.getClass().getSimpleName() + "' is not allowed as a "
+              + "Skylark value (checkTypeAllowedInSkylark() failed)");
     }
   }