On Fri, Aug 16, 2019 at 06:20:20PM -0700, Jason Merrill wrote:
On 8/16/19 2:29 PM, Marek Polacek wrote:
This patch is an attempt to fix the annoying -Wunused-but-set-* warnings that
tend to occur with constexpr if. When we have something like
template < typename T >
int f(T v){
if constexpr(sizeof(T) == sizeof(int)){
return v;
}else{
return 0;
}
}
and call f('a'), then the condition is false, meaning that we won't instantiate
the then-branch, as per tsubst_expr/IF_STMT:
17284 if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
17285 /* Don't instantiate the THEN_CLAUSE. */;
so we'll never get round to mark_exp_read-ing the decls used in the
then-branch, causing finish_function to emit "parameter set but not used"
warnings.
It's unclear how to best deal with this. Marking the decls DECL_READ_P while
parsing doesn't seem like a viable approach
Why not?
Well, while parsing, we're in a template and so the condition won't be
evaluated until tsubst_expr. So we can't tell which branch is dead.