Re: C++ PATCH for C++20 P0388R4 (conversions to arrays of unknown bounds) and CWG 1307 (c++/91364, c++/69531)

[Jonathan Wakely <jwakely at redhat dot com>]

On 07/10/19 14:56 -0400, Jason Merrill wrote:
> On 10/7/19 1:42 PM, Marek Polacek wrote:
> > @@ -7401,8 +7432,20 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
> >  	      error_at (loc, "cannot bind non-const lvalue reference of "
> >  			"type %qH to an rvalue of type %qI", totype, extype);
> >  	    else if (!reference_compatible_p (TREE_TYPE (totype), extype))
> > -	      error_at (loc, "binding reference of type %qH to %qI "
> > -			"discards qualifiers", totype, extype);
> > +	      {
> > +		/* If we're converting from T[] to T[N], don't talk
> > +		   about discarding qualifiers.  (Converting from T[N] to
> > +		   T[] is allowed by P0388R4.)  */
> > +		if (TREE_CODE (extype) == ARRAY_TYPE
> > +		    && TYPE_DOMAIN (extype) == NULL_TREE
> > +		    && TREE_CODE (TREE_TYPE (totype)) == ARRAY_TYPE
> > +		    && TYPE_DOMAIN (TREE_TYPE (totype)) != NULL_TREE)
> > +		  error_at (loc, "binding reference of type %qH to %qI "
> > +			    "discards array bounds", totype, extype);
>
> If we're converting to T[N], that would be adding, not discarding, 
> array bounds?

I don't think the diagnostic would be very good if we say "adds array
bounds" though. How about being consistent with the existing error for
similar cases?

a.cc:4:17: error: invalid initialization of reference of type ‘int (&)[3]’ from expression of type ‘int []’
  int (&b)[3] = a;
                ^