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%prec using undefined token

From: Florian Krohm
Subject: %prec using undefined token
Date: Tue, 29 Dec 2009 19:03:55 -0500
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I was debugging shift/reduce conflicts in a grammar and tracked
down the problem to a typo. I had misspelled a token name in a
%prec directive. The misspelled token name was an undefined token.
Like so:

%token INT
%left '-'
%left UMINUS


expr: term
    | expr '-' expr
    | '-' expr %prec UNINUS

term: INT
    | '(' expr ')'

bison 2.4.1 was silent about it even with --warnings=all

It would be consistent to issue an error about this situation.
After all one cannot use an undefined token elsewhere in a rule.
So why should this be allowed in %prec?


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