Do not allow BEGIN except the toplevel

include/yarp/diagnostic.h

@@ -42,6 +42,7 @@ typedef enum {
     YP_ERR_BEGIN_TERM,
     YP_ERR_BEGIN_UPCASE_BRACE,
     YP_ERR_BEGIN_UPCASE_TERM,
+    YP_ERR_BEGIN_UPCASE_TOPLEVEL,
     YP_ERR_BLOCK_PARAM_LOCAL_VARIABLE,
     YP_ERR_BLOCK_PARAM_PIPE_TERM,
     YP_ERR_BLOCK_TERM_BRACE,

src/diagnostic.c

@@ -77,6 +77,7 @@ static const char* const diagnostic_messages[YP_DIAGNOSTIC_ID_LEN] = {
     [YP_ERR_BEGIN_TERM]                         = "Expected an `end` to close the `begin` statement",
     [YP_ERR_BEGIN_UPCASE_BRACE]                 = "Expected a `{` after `BEGIN`",
     [YP_ERR_BEGIN_UPCASE_TERM]                  = "Expected a `}` to close the `BEGIN` statement",
+    [YP_ERR_BEGIN_UPCASE_TOPLEVEL]              = "BEGIN is permitted only at toplevel",
     [YP_ERR_BLOCK_PARAM_LOCAL_VARIABLE]         = "Expected a local variable name in the block parameters",
     [YP_ERR_BLOCK_PARAM_PIPE_TERM]              = "Expected the block parameters to end with `|`",
     [YP_ERR_BLOCK_TERM_BRACE]                   = "Expected a block beginning with `{` to end with `}`",

src/yarp.c

@@ -12145,6 +12145,10 @@ parse_expression_prefix(yp_parser_t *parser, yp_binding_power_t binding_power) {
             yp_statements_node_t *statements = parse_statements(parser, YP_CONTEXT_PREEXE);
 
             expect1(parser, YP_TOKEN_BRACE_RIGHT, YP_ERR_BEGIN_UPCASE_TERM);
+            yp_context_t context = parser->current_context->context;
+            if ((context != YP_CONTEXT_MAIN) && (context != YP_CONTEXT_PREEXE)) {
+                yp_diagnostic_list_append(&parser->error_list, keyword.start, keyword.end, YP_ERR_BEGIN_UPCASE_TOPLEVEL);
+            }
             return (yp_node_t *) yp_pre_execution_node_create(parser, &keyword, &opening, statements, &parser->previous);
         }
         case YP_TOKEN_KEYWORD_BREAK:

test/yarp/errors_test.rb

@@ -1302,6 +1302,13 @@ def test_alnum_delimiters
       assert_error_messages "%sXfooX", error_messages
     end
 
+    def test_begin_at_toplevel
+      source = "def foo; BEGIN {}; end"
+      assert_errors expression(source), source, [
+        ["BEGIN is permitted only at toplevel", 9..14],
+      ]
+    end
+
     def test_numbered_parameters_in_block_arguments
       source = "foo { |_1| }"
       assert_errors expression(source), source, [