https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88355
Bug ID: 88355
Summary: [c++20] Placeholder non-type template argument type
deduction fails with custom types
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: emmanuel.le-tr...@cnrs-orleans.fr
Target Milestone: ---
Deducing the type of a non-type template parameter containing a placeholder
fails with custom types.
$ g++-9 -v
Using built-in specs.
COLLECT_GCC=g++-9
COLLECT_LTO_WRAPPER=/home/manu/system/opt/gcc-9/libexec/gcc/x86_64-pc-linux-gnu/9.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-9/configure --prefix=/home/manu/system/opt/gcc-9
--program-suffix=-9 : (reconfigured) ../gcc-9/configure
--prefix=/home/manu/system/opt/gcc-9 --program-suffix=-9
Thread model: posix
gcc version 9.0.0 20181127 (experimental) (GCC)
$ cat bug_2.cpp
#include <type_traits> // for std::is_same_v
struct T {};
template <auto value>
struct U {};
template <auto value>
void f (U <value>) {}
int main ()
{
constexpr T t;
f (U<1>{}); // OK
f<t> (U<t>{}); // OK
// This works:
auto x = t;
static_assert (std::is_same_v <decltype (x), T>); // OK
// This should work as well, as per [temp.arg.nontype], see below
f (U<t>{}); // Error
}
$ g++-9 -std=c++2a -Wall -Wextra bug_2.cpp -o bug_2
bug_2.cpp: In function ‘int main()’:
bug_2.cpp:20:21: error: no matching function for call to ‘f(U<T()>)’
20 | f (U<t>{}); // Error
| ^
bug_2.cpp:9:10: note: candidate: ‘template<auto value> void f(U<value>)’
9 | void f (U <value>) {}
| ^
bug_2.cpp:9:10: note: template argument deduction/substitution failed:
bug_2.cpp:20:21: note: mismatched types ‘T’ and ‘const T’
20 | f (U<t>{}); // Error
| ^
make: *** [<builtin>: bug_2] Error 1
The standardese in the latest draft is:
---8<---
12.3.2 Template non-type arguments [temp.arg.nontype]
If the type T of a template-parameter (12.1) contains a placeholder type
(9.1.7.5) or a placeholder for a deduced class type (9.1.7.6), the type of the
parameter is the type deduced for the variable x in the invented declaration
T x = template-argument ;
--->8---
