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.

From-SVN: r195189