[Bug c++/55663] [C++11] Alias template combined with constexpr function is considered non-const

["dodji at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55663

--- Comment #5 from Dodji Seketeli <dodji at gcc dot gnu.org> 2013-01-15 09:12:42 UTC ---
Author: dodji
Date: Tue Jan 15 09:12:30 2013
New Revision: 195189

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195189
Log:
PR c++/55663 - constexpr function templ instantiation

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) (using instantiate_alias_template) 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 it constexpr-ness b/c we just look at its TREE_CONSTANT.

At that point, the_truth<int> should have been folded, and it's not,
because instantiate_alias_template forgets to call
coerce_template_parms on its arguments.

Fixed thus, bootstapped and tested on x86_64-unknown-linux-gnu against
trunk.

gcc/cp/

    PR c++/55663
    * pt.c (coerce_innermost_template_parms): New static function.
    (instantiate_alias_template):  Use it here.

gcc/testsuite/

    PR c++/55663
    * g++.dg/cpp0x/alias-decl-31.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/alias-decl-31.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog