PR c++/55311 - Cannot specialize alias template with arg of type array of char

PR c++/55311 - Cannot specialize alias template with arg of type array of char

Consider this test case:

     1	template <const char *const C, typename T>
     2	struct A
     3	{};
     4
     5	struct B {};
     6
     7	extern constexpr char HELLO_WORLD[] = "hello world";
     8
     9	A<HELLO_WORLD, B> g; // <-- This works fine
    10
    11	template <typename T>
    12	using PartiallySpecialized = A<HELLO_WORLD, T>;  // <-- This fails
    13

At line 12 G++ fails to instantiate the alias template that has a
string variable initialized with a string literal, with the error
message:

    test.cc:12:46: error: ‘"hello world"’ is not a valid template argument of type ‘const char*’ because ‘"hello world"’ is not a variable
     using PartiallySpecialized = A<HELLO_WORLD, T>;  // <-- This fails
                                              ^

Note that instantiating the template A at line 9 with the same
arguments as in the problematic case above works.

This happens in the context of lookup_template_class_1, when it handles
the alias template instantiation A<HELLO_WORLD, T> and thus passes the
VAR_DECL for HELLO_WORLD to convert_nontype_argument.

Note that from there decay_conversion replaces the the VAR_DECL with
its STRING_CST initializer[1].  Latter on, convert_nontype_argument
checks that the HELLO_WORLD constant it received as argument was
indeed a VAR_DECL:

      else
	{
	  tree decl;

	  decl = ((TREE_CODE (expr) == ADDR_EXPR)
		  ? TREE_OPERAND (expr, 0) : expr);
	  if (TREE_CODE (decl) != VAR_DECL)
	    {
	      error ("%qE is not a valid template argument of type %qT "
		     "because %qE is not a variable",
		     expr, type, decl);
	      return NULL_TREE;
	    }

But the issue is, that VAR_DECL has been replaced by STRING_CST, so
the last 'if' above fails.

The fix is to teach decay_conversion to return the address of array,
rather than returning its initializer.

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

gcc/cp/

	PR c++/55311
	* pt.c (decay_conversion): Do not return the initializer of an array.

gcc/testsuite/

	PR c++/55311
	* g++.dg/cpp0x/alias-decl-30.C: New test.
	* g++.dg/init/array21.C: New test.

From-SVN: r194961