bison/yacc: shift/reduce conflict using %prec for composition

Would someone explain why a simple grammar such as this:


%left FUNCTION


%%


expr: 'A'
        | expr expr %prec FUNCTION
;


causes a shift/reduce conflict in bison/yacc? Here is how I understand
my generated parser will work: all input must consist of a series of
the terminal 'A', which will be identified as an expr; when two exprs
are found adjacent, i.e., two 'A's, they will be reduced by the
left-to-right precedence declared by the %prec modifier. I thus see no
reason for a shift/reduce conflict.
[It's because precedences are attached to tokens, and there's no tokens
in that rule. -John]

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