https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97852
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think this is valid, t cannot show up in a constexpr.
ICC rejects this with a decent error message too:
<source>(3): error: a parameter is not allowed
And if we use foo MSVC rejects it:
<source>(7): error C2672: 'foo': no matching overloaded function found
<source>(7): error C2770: invalid explicit template argument(s) for
'S<int,t,N...> foo(T)'
<source>(3): note: see declaration of 'foo'
That is:
template <typename T, T...> struct S {};
template <typename T, long... N> auto foo(T t) -> S<decltype(0), (t, N)...>;
auto g(void)
{
return foo<int, 1>(100);
}
Note I don't think the original is valid either.
Take this C++17 code:
template <typename T, T...> struct S {};
constexpr int f(int, long) { return 0;}
template <typename T, long... N> constexpr auto foo(int t)
{
S<decltype(0), f(t, N)...> t1;
return t1;
}
auto g(void)
{
return foo<int, 1>(100);
}
---- CUT ----
All compilers reject this above code as invalid.
GCC with:
<source>: In instantiation of 'constexpr auto foo(int) [with T = int; long int
...N = {1}]':
<source>:13:23: required from here
<source>:7:32: error: 't' is not a constant expression
7 | S<decltype(0), f(t, N)...> t1;
| ^~
<source>:7:21: note: in template argument for type 'int'
7 | S<decltype(0), f(t, N)...> t1;
| ~^~~~~~
<source>:7:32: error: 't' is not a constant expression
7 | S<decltype(0), f(t, N)...> t1;
| ^~
<source>:7:21: note: in template argument for type 'int'
7 | S<decltype(0), f(t, N)...> t1;
| ~^~~~~~
Clang with
<source>:7:20: error: non-type template argument is not a constant expression
S<decltype(0), f(t, N)...> t1;
^~~~~~~
<source>:13:12: note: in instantiation of function template specialization
'foo<int, 1L>' requested here
return foo<int, 1>(100);
^
<source>:7:22: note: function parameter 't' with unknown value cannot be used
in a constant expression
S<decltype(0), f(t, N)...> t1;
^
<source>:5:57: note: declared here
template <typename T, long... N> constexpr auto foo(int t)
^
this is similar to workaround 2 but with an instantiation to show that
workaround 2 does not work really.
