On Thu, Feb 04, 2021 at 10:59:21AM -0500, Jason Merrill wrote: > On 2/3/21 7:03 PM, Marek Polacek wrote: > > Since most of volatile is deprecated in C++20, we are required to warn > > for compound assignments to volatile variables and so on. But here we > > have > > > > volatile int x, y, z; > > (b ? x : y) = 1; > > > > and we shouldn't warn, because simple assignments like x = 24; should > > not provoke the warning when they are a discarded-value expression. > > > > We warn here because when ?: is used as an lvalue, we transform it in > > cp_build_modify_expr/COND_EXPR from (a ? b : c) = rhs to > > > > (a ? (b = rhs) : (c = rhs)) > > > > and build_conditional_expr then calls mark_lvalue_use for the new > > artificial assignments > > Hmm, that seems wrong; the ?: expression itself does not use lvalue operands > any more than ',' does. I notice that removing those mark_lvalue_use calls > doesn't regress Wunused-var-10.c, which was added with them in r160289.
The mark_lvalue_use calls didn't strike me as wrong because [expr.cond]/7 says that lvalue-to-rvalue conversion is performed on the second and third operands. With those mark_lvalue_use calls removed, we'd not issue the warning for (b ? (x = 2) : y) = 1; (b ? x : (y = 5)) = 1; which I think we want (and clang++ warns here too). > > --- a/gcc/cp/expr.c > > +++ b/gcc/cp/expr.c > > @@ -228,7 +228,8 @@ mark_use (tree expr, bool rvalue_p, bool read_p, > > && !cp_unevaluated_operand > > && (TREE_THIS_VOLATILE (lhs) > > || CP_TYPE_VOLATILE_P (TREE_TYPE (lhs))) > > - && !TREE_THIS_VOLATILE (expr)) > > + && !TREE_THIS_VOLATILE (expr) > > + && warn_volatile) > > Or you could check the return value of the warning? Works too. Happy to change that. Marek
