compiler: check duplicate implicit indexes in slices/arrays

    
    Fixes golang/go#28186.
    
    Reviewed-on: https://go-review.googlesource.com/c/160832

From-SVN: r268554

gcc/go/gofrontend/MERGE

-047b0aa6a29d46fde99b3e5823339ac8866f797c
+347628daf153baf3034b61b2abb4ec39e2ab37c8
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -14244,6 +14244,13 @@ Composite_literal_expression::lower_array(Type* type)
 
       if (index_expr == NULL)
 	{
+	  if (std::find(indexes->begin(), indexes->end(), index)
+	      != indexes->end())
+	    {
+	      go_error_at(val->location(),
+			  "duplicate value for index %lu", index);
+	      return Expression::make_error(location);
+	    }
 	  if (!indexes->empty())
 	    indexes->push_back(index);
 	}