https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115614
Bug ID: 115614
Summary: Invalid (?) template substitution on variadic
constrained packs
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: cohenarthur at gcc dot gnu.org
Target Milestone: ---
GCC 14.1 rejects code that has been accepted by clang since version 15, which
makes me think it should be accepted.
```
#include <stddef.h>
#include <source_location>
#include <type_traits>
#include <utility>
template <typename... Ts>
struct l {};
template <typename T>
struct c {
using type = T;
};
template <typename, size_t>
concept e = true;
template <size_t... i, size_t... j, size_t... k>
auto subs0(std::index_sequence<i...>,
std::index_sequence<j...>,
std::index_sequence<k...>) {
return []<e<i>... prefix,
e<j>... infix,
e<k>... suffix>
(prefix..., infix..., suffix...) {
return l<typename infix::type...>{};
};
}
template <size_t a, size_t b, typename... Ts>
auto subs(l<Ts...> ls) {
return subs0(std::make_index_sequence<a>{},
std::make_index_sequence<b - a>{},
std::make_index_sequence<sizeof...(Ts) - b>{}
)(c<Ts>{}...);
}
template <size_t a, size_t b, typename Ts>
using sub = decltype(subs<a, b>(Ts{}));
void x(sub<0, 3, l<int, int*, int**>>) {}
void x(sub<0, 2, l<int, int*, int**>>) {}
void x(sub<1, 2, l<int, int*, int**>>) {}
void x(sub<1, 3, l<int, int*, int**>>) {}
```
clang shows no errors, whereas g++ outputs the following:
```
<source>: In instantiation of 'auto subs(l<Ts ...>) [with long unsigned int a =
0; long unsigned int b = 3; Ts = {int, int*, int**}]':
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 0; long unsigned int b = 3; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:41:36: required from here
41 | void x(sub<0, 3, l<int, int*, int**>>) {}
| ^~
<source>:35:16: error: no match for call to '(subs0<, 0, 1,
2>(std::index_sequence<>, std::index_sequence<0, 1, 2>,
std::index_sequence<>)::<lambda(prefix ..., infix ..., suffix ...)>) (c<int>,
c<int*>, c<int**>)'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:22:10: note: candidate: 'template<class ... prefix, class ... infix,
class ... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>))
&& ((... && e<suffix, k>)) subs0<, 0, 1, 2>(std::index_sequence<>,
std::index_sequence<0, 1, 2>, std::index_sequence<>)::<lambda(prefix ..., infix
..., suffix ...)>'
22 | return []<e<i>... prefix,
| ^
<source>:22:10: note: template argument deduction/substitution failed:
<source>:22:10: note: constraints not satisfied
<source>: In substitution of 'template<class ... prefix, class ... infix, class
... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>)) &&
((... && e<suffix, k>)) subs0<, 0, 1, 2>(std::index_sequence<>,
std::index_sequence<0, 1, 2>, std::index_sequence<>)::<lambda(prefix ..., infix
..., suffix ...)> [with prefix = {}; infix = {0, 1, 2}; suffix = {}]':
<source>:35:16: required from 'auto subs(l<Ts ...>) [with long unsigned int a
= 0; long unsigned int b = 3; Ts = {int, int*, int**}]'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 0; long unsigned int b = 3; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:41:36: required from here
41 | void x(sub<0, 3, l<int, int*, int**>>) {}
| ^~
<source>:22:10: required by the constraints of 'template<long unsigned int
...i, long unsigned int ...j, long unsigned int ...k> template<class ...
prefix, class ... infix, class ... suffix> requires ((... && e<prefix, i>)) &&
((... && e<infix, j>)) && ((... && e<suffix, k>)) subs0(std::index_sequence<i
...>, std::index_sequence<j ...>, std::index_sequence<k ...>)::<lambda(prefix
..., infix ..., suffix ...)>'
<source>:35:16: error: mismatched argument pack lengths while expanding
'e<infix, j>'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>: In instantiation of 'auto subs(l<Ts ...>) [with long unsigned int a =
0; long unsigned int b = 2; Ts = {int, int*, int**}]':
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 0; long unsigned int b = 2; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:42:36: required from here
42 | void x(sub<0, 2, l<int, int*, int**>>) {}
| ^~
<source>:35:16: error: no match for call to '(subs0<, 0, 1,
0>(std::index_sequence<>, std::index_sequence<0, 1>,
std::index_sequence<0>)::<lambda(prefix ..., infix ..., suffix ...)>) (c<int>,
c<int*>, c<int**>)'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:22:10: note: candidate: 'template<class ... prefix, class ... infix,
class ... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>))
&& ((... && e<suffix, k>)) subs0<, 0, 1, 0>(std::index_sequence<>,
std::index_sequence<0, 1>, std::index_sequence<0>)::<lambda(prefix ..., infix
..., suffix ...)>'
22 | return []<e<i>... prefix,
| ^
<source>:22:10: note: template argument deduction/substitution failed:
<source>:22:10: note: constraints not satisfied
<source>: In substitution of 'template<class ... prefix, class ... infix, class
... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>)) &&
((... && e<suffix, k>)) subs0<, 0, 1, 0>(std::index_sequence<>,
std::index_sequence<0, 1>, std::index_sequence<0>)::<lambda(prefix ..., infix
..., suffix ...)> [with prefix = {}; infix = {0, 1}; suffix = {0}]':
<source>:35:16: required from 'auto subs(l<Ts ...>) [with long unsigned int a
= 0; long unsigned int b = 2; Ts = {int, int*, int**}]'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 0; long unsigned int b = 2; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:42:36: required from here
42 | void x(sub<0, 2, l<int, int*, int**>>) {}
| ^~
<source>:22:10: required by the constraints of 'template<long unsigned int
...i, long unsigned int ...j, long unsigned int ...k> template<class ...
prefix, class ... infix, class ... suffix> requires ((... && e<prefix, i>)) &&
((... && e<infix, j>)) && ((... && e<suffix, k>)) subs0(std::index_sequence<i
...>, std::index_sequence<j ...>, std::index_sequence<k ...>)::<lambda(prefix
..., infix ..., suffix ...)>'
<source>:35:16: error: mismatched argument pack lengths while expanding
'e<infix, j>'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:42:6: error: redefinition of 'void x(int)'
42 | void x(sub<0, 2, l<int, int*, int**>>) {}
| ^
<source>:41:6: note: 'void x(int)' previously defined here
41 | void x(sub<0, 3, l<int, int*, int**>>) {}
| ^
<source>: In instantiation of 'auto subs(l<Ts ...>) [with long unsigned int a =
1; long unsigned int b = 2; Ts = {int, int*, int**}]':
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 1; long unsigned int b = 2; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:43:36: required from here
43 | void x(sub<1, 2, l<int, int*, int**>>) {}
| ^~
<source>:35:16: error: no match for call to '(subs0<0, 0,
0>(std::index_sequence<0>, std::index_sequence<0>,
std::index_sequence<0>)::<lambda(prefix ..., infix ..., suffix ...)>) (c<int>,
c<int*>, c<int**>)'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:22:10: note: candidate: 'template<class ... prefix, class ... infix,
class ... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>))
&& ((... && e<suffix, k>)) subs0<0, 0, 0>(std::index_sequence<0>,
std::index_sequence<0>, std::index_sequence<0>)::<lambda(prefix ..., infix ...,
suffix ...)>'
22 | return []<e<i>... prefix,
| ^
<source>:22:10: note: template argument deduction/substitution failed:
<source>:22:10: note: constraints not satisfied
<source>: In substitution of 'template<class ... prefix, class ... infix, class
... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>)) &&
((... && e<suffix, k>)) subs0<0, 0, 0>(std::index_sequence<0>,
std::index_sequence<0>, std::index_sequence<0>)::<lambda(prefix ..., infix ...,
suffix ...)> [with prefix = {0}; infix = {0}; suffix = {0}]':
<source>:35:16: required from 'auto subs(l<Ts ...>) [with long unsigned int a
= 1; long unsigned int b = 2; Ts = {int, int*, int**}]'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 1; long unsigned int b = 2; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:43:36: required from here
43 | void x(sub<1, 2, l<int, int*, int**>>) {}
| ^~
<source>:22:10: required by the constraints of 'template<long unsigned int
...i, long unsigned int ...j, long unsigned int ...k> template<class ...
prefix, class ... infix, class ... suffix> requires ((... && e<prefix, i>)) &&
((... && e<infix, j>)) && ((... && e<suffix, k>)) subs0(std::index_sequence<i
...>, std::index_sequence<j ...>, std::index_sequence<k ...>)::<lambda(prefix
..., infix ..., suffix ...)>'
<source>:35:16: error: mismatched argument pack lengths while expanding
'e<prefix, i>'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:43:6: error: redefinition of 'void x(int)'
43 | void x(sub<1, 2, l<int, int*, int**>>) {}
| ^
<source>:41:6: note: 'void x(int)' previously defined here
41 | void x(sub<0, 3, l<int, int*, int**>>) {}
| ^
<source>: In instantiation of 'auto subs(l<Ts ...>) [with long unsigned int a =
1; long unsigned int b = 3; Ts = {int, int*, int**}]':
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 1; long unsigned int b = 3; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:44:36: required from here
44 | void x(sub<1, 3, l<int, int*, int**>>) {}
| ^~
<source>:35:16: error: no match for call to '(subs0<0, 0,
1>(std::index_sequence<0>, std::index_sequence<0, 1>,
std::index_sequence<>)::<lambda(prefix ..., infix ..., suffix ...)>) (c<int>,
c<int*>, c<int**>)'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:22:10: note: candidate: 'template<class ... prefix, class ... infix,
class ... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>))
&& ((... && e<suffix, k>)) subs0<0, 0, 1>(std::index_sequence<0>,
std::index_sequence<0, 1>, std::index_sequence<>)::<lambda(prefix ..., infix
..., suffix ...)>'
22 | return []<e<i>... prefix,
| ^
<source>:22:10: note: template argument deduction/substitution failed:
<source>:22:10: note: constraints not satisfied
<source>: In substitution of 'template<class ... prefix, class ... infix, class
... suffix> requires ((... && e<prefix, i>)) && ((... && e<infix, j>)) &&
((... && e<suffix, k>)) subs0<0, 0, 1>(std::index_sequence<0>,
std::index_sequence<0, 1>, std::index_sequence<>)::<lambda(prefix ..., infix
..., suffix ...)> [with prefix = {0}; infix = {0, 1}; suffix = {}]':
<source>:35:16: required from 'auto subs(l<Ts ...>) [with long unsigned int a
= 1; long unsigned int b = 3; Ts = {int, int*, int**}]'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:39:32: required by substitution of 'template<long unsigned int a,
long unsigned int b, class Ts> using sub = decltype (subs<a, b>(Ts{})) [with
long unsigned int a = 1; long unsigned int b = 3; Ts = l<int, int*, int**>]'
39 | using sub = decltype(subs<a, b>(Ts{}));
| ~~~~~~~~~~^~~~~~
<source>:44:36: required from here
44 | void x(sub<1, 3, l<int, int*, int**>>) {}
| ^~
<source>:22:10: required by the constraints of 'template<long unsigned int
...i, long unsigned int ...j, long unsigned int ...k> template<class ...
prefix, class ... infix, class ... suffix> requires ((... && e<prefix, i>)) &&
((... && e<infix, j>)) && ((... && e<suffix, k>)) subs0(std::index_sequence<i
...>, std::index_sequence<j ...>, std::index_sequence<k ...>)::<lambda(prefix
..., infix ..., suffix ...)>'
<source>:35:16: error: mismatched argument pack lengths while expanding
'e<prefix, i>'
32 | return subs0(std::make_index_sequence<a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 | std::make_index_sequence<b - a>{},
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
34 | std::make_index_sequence<sizeof...(Ts) - b>{}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
35 | )(c<Ts>{}...);
| ~^~~~~~~~~~~~
<source>:44:6: error: redefinition of 'void x(int)'
44 | void x(sub<1, 3, l<int, int*, int**>>) {}
| ^
<source>:41:6: note: 'void x(int)' previously defined here
41 | void x(sub<0, 3, l<int, int*, int**>>) {}
| ^
Compiler returned: 1
```
CE link for a quick comparison between GCC 14.1 and clang 15:
https://godbolt.org/z/YGfq5bG44
