On Wed, May 30, 2018 at 5:23 PM, Marek Polacek <[email protected]> wrote:
> We are failing to deduce the template parameter N here
>
> template <int N>
> void foo(const long int (&)[N]) {}
>
> void bar() {
> foo ({1,2,3});
> }
>
> because of the type mismatch; parm is long int (element type of the array),
> while arg is int (element type of {1, 2, 3}), and unify doesn't like that:
>
> 21789 /* We have already checked cv-qualification at the top of the
> 21790 function. */
> 21791 if (!same_type_ignoring_top_level_qualifiers_p (arg, parm))
> 21792 return unify_type_mismatch (explain_p, parm, arg);
>
> But since the parameter type is array, we should see if there exists an
> implicit conversion sequence for each element of the array from the
> corresponding element of the initializer list, and that's what I tried,
> and it seems to work.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-05-30 Marek Polacek <[email protected]>
>
> PR c++/85977
> * pt.c (unify): Handle the [over.ics.list]/6 case.
>
> + /* [over.ics.list]/6 says we should try an implicit conversion
> + from each list element to the corresponding array element
> + type. */
[over.ics.list] doesn't apply during template argument deduction, but
rather [temp.deduct.call].
> + if (TREE_CODE (parm) == ARRAY_TYPE)
> + {
> + tree x = perform_implicit_conversion (elttype, elt,
> complain);
Rather than check this immediately here, we want to do more or less
what unify_one_argument does:
if (strict != DEDUCE_EXACT
&& TYPE_P (parm) && !uses_deducible_template_parms (parm))
/* For function parameters with no deducible template parameters,
just return. We'll check non-dependent conversions later. */
return unify_success (explain_p);
so if elttype has no deducible template parms, don't do deduction from
the list elements at all.
And then we want to check convertibility of the elements in
type_unification_real, when we check convertibility of other function
parameters that don't involve template parameters:
/* DR 1391: All parameters have args, now check non-dependent
parms for
convertibility. */
Jason
