Re: C++ PATCH for c++/81676 - bogus -Wunused warnings in constexpr if

[Marek Polacek <polacek at redhat dot com>]

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.

I suppose we could still mark all the _DECLs in then/else branches as read,
in finish_if_stmt if IF_STMT_CONSTEXPR_P, but that seems really really ugly.

--
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA