warning: type clash on default action: <symp> != <>

Question

I faced this problen when I am trying to run .y file

Phase2.y:119.10-26: warning: type clash on default action: <symp> != <>

line 119 in .y file we have

`

factor : LPAREN exp RPAREN
       | INT_LITERAL {
            strcpy($$.type ,"int");
strcpy($$.name,"");
    
}

`

I saw simple explanation when I am trying to found something to help but not clear for me

any help? thank you

......................................

Answer 1

That rule contains two productions. The first production does not have a semantic action, so the default action is used. The default action is { $$ = $1; }

Bison tries to verify that the default action is correctly typed. In this case, $$ is factor, which you have probably declared to be %type <symp> factor. $1 is LPAREN, which you have presumably declared to be an untyped token (that is, a token with no semantic value). Both of those declarations are correct, but that means that $$ = $1; is nonsense; you can't assign $$ from a non-value.

You probably meant to use the expr as the value of the first production. It's $2, since expr is the second symbol on the right-hand side of the production. (You can also write it as $expr, which might be more clear.)

So the rule should read:

factor : LPAREN exp RPAREN { $$ = $2; }
       | INT_LITERAL {
            strcpy($$.type ,"int");
            strcpy($$.name,"");   
         }

Except that the semantic action for the second production is ignoring the value of the INT_LITERAL token, which seems unlikely to be correct.