[Bug c++/55311] Cannot specialize template parameter of type 'const char *const' in 'using' alias

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55311

--- Comment #4 from Dodji Seketeli <dodji at gcc dot gnu.org> 2013-01-07 
08:06:52 UTC ---
Author: dodji
Date: Mon Jan  7 08:06:46 2013
New Revision: 194961

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=194961
Log:
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But the issue is, that VAR_DECL has been replaced by STRING_CST, so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.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/alias-decl-30.C
Modified:
    trunk/gcc/cp/typeck.c
    trunk/gcc/testsuite/g++.dg/init/array21.C