PR c++/54875 - Error with alias template that resolves to an enum

Consider this short example:

     1	template<typename T>
     2	using AddConst = T const;
     3
     4	enum FwdEnum : int;
     5
     6	int main() {
     7	  AddConst<FwdEnum> *ptr = nullptr;
     8	}

At line 7, when we build the type for AddConst<FwdEnum> in
lookup_template_class_1, the resulting type is the enum FwdEnum.  This
confuses lookup_template_class_1 near the if below, wrongly making it
taking the branch and thus calling tsubst_enum while it shouldn't:

      if (TREE_CODE (t) == ENUMERAL_TYPE && !is_dependent_type)
	/* Now that the type has been registered on the instantiations
	   list, we set up the enumerators.  Because the enumeration
	   constants may involve the enumeration type itself, we make
	   sure to register the type first, and then create the
	   constants.  That way, doing tsubst_expr for the enumeration
	   constants won't result in recursive calls here; we'll find
	   the instantiation and exit above.  */
	tsubst_enum (template_type, t, arglist);

Before the alias template feature, the only reason why TREE_CODE (t)
== ENUMERAL_TYPE would be true is when lookup_template_class_1 is
called for an enum that is a member of a class template.  But that
condition can be also true for an alias template instantiation.

So I guess that condition should be changed to TREE_CODE
(template_type) == ENUMERAL_TYPE, to specifically detect the member
enum of a class template case.  Note that for the alias template
instantiation case above, template_type points to a TEMPLATE_TYPE_PARM
which name is AddConst.

This is what the patchlet below does.

Bootstrapped and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

	* pt.c (lookup_template_class_1): Look at the type of the
	potential member enum of class template to determine if we are
	actually substituting into a member enum of class template.

gcc/testsuite/

	* g++.dg/cpp0x/alias-decl-27.C: New test.

From-SVN: r193562