[Bug c++/35852] -Wconversion rejects bitwise negation and assignment, cannot cast around

["manu at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from manu at gcc dot gnu dot org  2008-08-01 17:54 -------
To be honest, I am not sure what deserves a warning and what not in this
testcase.

FOO is 19, ~FOO is -20. Therefore 
ushort x = ~FOO seems to deserve a warning (with -Wsing-conversion at least).

x = x & -20 is handled as x = (int)x & -20. If x == 0xFFFF, this seems like the
case above and probably deserves a warning.

lv = lv & ~rv is seen as:

 <bit_and_expr 
    type <integer_type int public SI
        size <integer_cst constant 32>
        unit size <integer_cst  constant 4>
    arg 0 <nop_expr 0x2aaaab238940 type <integer_type  int>

        arg 0 <indirect_ref  type <integer_type  short unsigned int>
            arg 0 <parm_decl  lv>>>
    arg 1 <bit_not_expr  type <integer_type  int>
        arg 0 <nop_expr  type <integer_type  int>
            arg 0 <indirect_ref  type <integer_type  short unsigned int>
                tree_0 arg 0 <parm_decl  rv>>>>>

That is, we would need to peek inside each operand and recursively check
whether the expressions/conversions are safe. I don't know if this is feasible
at all and I personally don't know at this moment how to implement it properly.

On the other hand, in mainline, I don't get any warnings for:

        x = x & ushort(~FOO);   // -Wconversion
        x = x & ushort(~ushort(FOO));   // -Wconversion
        x &= static_cast<ushort>(~FOO);         // -Wconversion

So that seems fixed.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35852