On Tue, Jan 8, 2013 at 7:58 AM, Dodji Seketeli <do...@redhat.com> wrote:
> Hello,
>
> Consider the example of the problem report
>
> 1 template <typename>
> 2 constexpr bool the_truth () { return true; }
> 3
> 4 template <bool>
> 5 struct Takes_bool { };
> 6
> 7 template<bool B>
> 8 using Alias = Takes_bool<B>;
> 9
> 10 template<typename T>
> 11 struct test { using type = Alias<the_truth<T>()>; };
> 12
> 13 int main () {
> 14 test<int> a;
> 15
> 16 return 0;
> 17 }
>
> that yields the error:
>
> test.cc: In substitution of ‘template<bool B> using Alias = Takes_bool<B>
> [with bool B = the_truth<int>()]’:
> test.cc:11:51: required from ‘struct test<int>’
> test.cc:14:13: required from here
> test.cc:11:51: error: integral expression ‘the_truth<int>()’ is not
> constant
> struct test { using type = Alias<the_truth<T>()>; };
>
> I think the issue happens in the course of instantiating test<int> at
> line 14, when we look into instantiating Alias<the_truth<T>()> (at
> line 11), with T = int.
>
> There, when we check the argument 'the_truth<int>()' to see if it
> actually is a constant expression, in check_instantiated_arg, we fail
> to recognize its constexpr-ness b/c we just look at its TREE_CONSTANT.
Thanks for the detective work!
We already have various predicates to test for constant
expressions so I am uneasy to add yet another one.
What we do no need -- which I already suggested -- is a
predicate to test valid non-type template arguments.
For example, we already have the predicate verify_constant
and reduced_constant_expression_p, require_potential_constant_expression.
I think reduced_constant_expression_p is what you want.
-- Gaby
