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Why do some versions of bison require {} here?

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In SDCC.y, there are rules:

program
   : external_definition
   | program external_definition
   ;

external_definition
   : function_definition
        {
          // blockNo = 0;
        }
   | declaration
        {
          ignoreTypedefType = 0;
          if ($1 && $1->type && IS_FUNC($1->type))
            {
              /* The only legal storage classes for
               * a function prototype (declaration)
               * are extern and static. extern is the
               * default. Thus, if this function isn't
               * explicitly marked static, mark it
               * extern.
               */
              if ($1->etype && IS_SPEC($1->etype) && !SPEC_STAT($1->etype))
                {
                  SPEC_EXTR($1->etype) = 1;
                }
            }
          addSymChain (&$1);
          allocVariables ($1);
          cleanUpLevel (SymbolTab, 1);
        }
   | addressmod
   ;

which has always worked for me without problems on all yacc / bison I
tried. But sporadically, users reported that this failed for them, and
they had to change the last two lines to:

   | addressmod
        {
        }
   ;

for their yacc / bison to accept the rule.
What is happening here? Why is the {} required? Why only on the last
rule, while the first one is okay?

Philipp
[Adding the empty action forces bison to reduce the rule rather than just
shifting and saving state for later.  I couldn't guess why that would matter
in this case.  Are there precedence rules?  With your change does the grammar compile
cleanly or does it have conflicts? -John]

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Thread

Why do some versions of bison require {} here? Philipp Klaus Krause <pkk@spth.de> - 2019-01-01 10:55 +0100
  Re: Why do some versions of bison require {} here? Kaz Kylheku <157-073-9834@kylheku.com> - 2019-01-02 19:29 +0000
  Re: Why do some versions of bison require {} here? Philipp Klaus Krause <pkk@spth.de> - 2019-01-03 09:15 +0100
    Re: Why do some versions of bison require {} here? Kaz Kylheku <157-073-9834@kylheku.com> - 2019-01-04 03:02 +0000

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