Throw a static error when lvalue of an augmented assignment is a list.

RELNOTES:
  When the lvalue of an augmented assignment is a list, we now throw an error
  before evaluating the code (e.g. `a, b += 2, 3`).
PiperOrigin-RevId: 165906611

1 files changed, 10 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
index 4415a85ce4..6f5eb9c401 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ValidationEnvironment.java
@@ -158,6 +158,16 @@ public final class ValidationEnvironment extends SyntaxTreeVisitor {
     super.visit(node);
   }
 
+  @Override
+  public void visit(AugmentedAssignmentStatement node) {
+    if (node.getLValue().getExpression() instanceof ListLiteral) {
+      throw new ValidationException(
+          node.getLocation(), "cannot perform augmented assignment on a list or tuple expression");
+    }
+    // Other bad cases are handled when visiting the LValue node.
+    super.visit(node);
+  }
+
   /** Returns true if the current block is the top level i.e. has no parent. */
   private boolean isTopLevel() {
     return block.parent == null;