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

Chris Lattner clattner at apple.com
Tue Aug 28 10:38:22 PDT 2007

On Aug 28, 2007, at 10:21 AM, Ted Kremenek 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.


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