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But the issue is, that VAR_DECL has been replaced by STRING_CST, so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

gcc/cp/typeck.c

@@ -1880,19 +1880,25 @@ decay_conversion (tree exp, tsubst_flags_t complain)
       return error_mark_node;
     }
 
-  /* FIXME remove? at least need to remember that this isn't really a
-     constant expression if EXP isn't decl_constant_var_p, like with
-     C_MAYBE_CONST_EXPR.  */
-  exp = decl_constant_value_safe (exp);
-  if (error_operand_p (exp))
-    return error_mark_node;
+  code = TREE_CODE (type);
+
+  /* For an array decl decay_conversion should not try to return its
+     initializer.  */
+  if (code != ARRAY_TYPE)
+    {
+      /* FIXME remove? at least need to remember that this isn't really a
+	 constant expression if EXP isn't decl_constant_var_p, like with
+	 C_MAYBE_CONST_EXPR.  */
+      exp = decl_constant_value_safe (exp);
+      if (error_operand_p (exp))
+	return error_mark_node;
+    }
 
   if (NULLPTR_TYPE_P (type) && !TREE_SIDE_EFFECTS (exp))
     return nullptr_node;
 
   /* build_c_cast puts on a NOP_EXPR to make the result not an lvalue.
      Leave such NOP_EXPRs, since RHS is being used in non-lvalue context.  */
-  code = TREE_CODE (type);
   if (code == VOID_TYPE)
     {
       if (complain & tf_error)

gcc/testsuite/g++.dg/cpp0x/alias-decl-30.C

@@ -0,0 +1,15 @@
+// Origin PR c++/55311
+// { dg-do compile { target c++11 } }
+
+template <const char *const C, typename T>
+struct A
+{};
+
+struct B {};
+
+extern constexpr char HELLO_WORLD[] = "hello world";
+
+A<HELLO_WORLD, B> g; // <-- This works fine
+
+template <typename T>
+using PartiallySpecialized = A<HELLO_WORLD, T>;  // <-- This fails

gcc/testsuite/g++.dg/init/array21.C

@@ -3,5 +3,5 @@
 void foo()
 {
   const int x[] = 0; // { dg-error "initializer" }
-  ++x;
+  ++x; // { dg-error "read-only|operand" }
 }