[cfe-dev] "expression result unused" in comma expression

[Ted Kremenek]

On Aug 28, 2007, at 10:38 AM, Chris Lattner wrote:

>> A test case to the frontend generated the following unexpected  
>> warning:
>>
>>   warning: expression result unused
>>     x = ++x, ++y, y+2;
>>     ^~~~~~~~~~~~~~~~~
>>
>>   int comma_in_assignment(int x, int y) {
>>     x = ++x, ++y, y+2;
>>     return x;
>>   }
>>
>> Removing the "++x" also generates the same warning.  Any thoughts?   
>> Is
>> this desired behavior?
>
> Note that this is parsed as:
>  (x = ++x), ++y, y+2;
>
> $ clang t2.c -parse-ast-dump
> int comma_in_assignment(int x, int y)
> (CompoundStmt 0x2505e50
>  (BinaryOperator 0x2505dc0 'int' ','
>    (BinaryOperator 0x2505d40 'int' ','
>      (BinaryOperator 0x2505ce0 'int' '='
>        (DeclRefExpr 0x2505c80 'int' Decl='x' 0x2505c10)
>        (UnaryOperator 0x2505cc0 'int' prefix '++'
>          (DeclRefExpr 0x2505ca0 'int' Decl='x' 0x2505c10)))
>      (UnaryOperator 0x2505d20 'int' prefix '++'
>        (DeclRefExpr 0x2505d00 'int' Decl='y' 0x2505c40)))
>    (BinaryOperator 0x2505da0 'int' '+'
>      (DeclRefExpr 0x2505d60 'int' Decl='y' 0x2505c40)
>      (IntegerLiteral 0x2505d80 'int' 2)))
>  (ReturnStmt 0x2505e00
>    (DeclRefExpr 0x2501490 'int' Decl='x' 0x2505c10)))
>
> So the y+2 is unused.  I will fix the diagnostic to be more precise.

Thanks for pointing this out.  On a related matter, it does appear  
that this check doesn't handle statement expressions (unless I'm,  
quite possibly, also reading this wrong!):

int maxval_stmt_expr(int x, int y) {
   return ({int _a = x, _b = y; _a > _b ? _a : _b; });
}

warning: expression result unused
   return ({int _a = x, _b = y; _a > _b ? _a : _b; });
                                ^~~~~~~~~~~~~~~~~

(it's pointing to the ternary expression)