[Bug c++/55311] Cannot specialize template parameter of type 'const char *const' in 'using' alias
dodji at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Mon Jan 7 08:07:00 GMT 2013
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But the issue is, that VAR_DECL has been replaced by STRING_CST, so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
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