Deduction guides and variadic class templates with variadic template constructors - mismatched argument pack...
Consider the following class definition and deduction guide:
template <typename... Ts>
struct foo : Ts...
{
template <typename... Us>
foo(Us&&... us) : Ts{us}... { }
};
template <typename... Us>
foo(Us&&... us) -> foo<Us...>;
If I try to instantiate foo with explicit template arguments, the code compiles correctly:
foo<bar> a{bar{}}; // ok
If I try to instantiate foo through the deduction guide...
foo b{bar{}};
g++7 produces a compiler error:
prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
prog.cc:15:16: required from here
prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
foo(Us... us) : Ts{us}... { }
^~~
clang++5 explodes:
#0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
#1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
#2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
#3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
...
clang-5.0: error: unable to execute command: Segmentation fault
live example on wandbox
While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?
c++ c++17 template-deduction
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
add a comment |
Consider the following class definition and deduction guide:
template <typename... Ts>
struct foo : Ts...
{
template <typename... Us>
foo(Us&&... us) : Ts{us}... { }
};
template <typename... Us>
foo(Us&&... us) -> foo<Us...>;
If I try to instantiate foo with explicit template arguments, the code compiles correctly:
foo<bar> a{bar{}}; // ok
If I try to instantiate foo through the deduction guide...
foo b{bar{}};
g++7 produces a compiler error:
prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
prog.cc:15:16: required from here
prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
foo(Us... us) : Ts{us}... { }
^~~
clang++5 explodes:
#0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
#1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
#2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
#3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
...
clang-5.0: error: unable to execute command: Segmentation fault
live example on wandbox
While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?
c++ c++17 template-deduction
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
3
Specifically, the problem seems to be thatTsis not correctly deduced
– Ben Steffan
Apr 15 '17 at 20:50
add a comment |
Consider the following class definition and deduction guide:
template <typename... Ts>
struct foo : Ts...
{
template <typename... Us>
foo(Us&&... us) : Ts{us}... { }
};
template <typename... Us>
foo(Us&&... us) -> foo<Us...>;
If I try to instantiate foo with explicit template arguments, the code compiles correctly:
foo<bar> a{bar{}}; // ok
If I try to instantiate foo through the deduction guide...
foo b{bar{}};
g++7 produces a compiler error:
prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
prog.cc:15:16: required from here
prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
foo(Us... us) : Ts{us}... { }
^~~
clang++5 explodes:
#0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
#1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
#2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
#3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
...
clang-5.0: error: unable to execute command: Segmentation fault
live example on wandbox
While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?
c++ c++17 template-deduction
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
Consider the following class definition and deduction guide:
template <typename... Ts>
struct foo : Ts...
{
template <typename... Us>
foo(Us&&... us) : Ts{us}... { }
};
template <typename... Us>
foo(Us&&... us) -> foo<Us...>;
If I try to instantiate foo with explicit template arguments, the code compiles correctly:
foo<bar> a{bar{}}; // ok
If I try to instantiate foo through the deduction guide...
foo b{bar{}};
g++7 produces a compiler error:
prog.cc: In instantiation of 'foo<Ts>::foo(Us ...) [with Us = {bar}; Ts = {}]':
prog.cc:15:16: required from here
prog.cc:5:27: error: mismatched argument pack lengths while expanding 'Ts'
foo(Us... us) : Ts{us}... { }
^~~
clang++5 explodes:
#0 0x0000000001944af4 PrintStackTraceSignalHandler(void*) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944af4)
#1 0x0000000001944dc6 SignalHandler(int) (/opt/wandbox/clang-head/bin/clang-5.0+0x1944dc6)
#2 0x00007fafb639a390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
#3 0x0000000003015b30 clang::Decl::setDeclContext(clang::DeclContext*) (/opt/wandbox/clang-head/bin/clang-5.0+0x3015b30)
...
clang-5.0: error: unable to execute command: Segmentation fault
live example on wandbox
While clang++ is definitely bugged (reported as issue #32673), is g++ correct in rejecting my code? Is my code ill-formed?
c++ c++17 template-deduction
c++ c++17 template-deduction
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
edited Apr 15 '17 at 20:41
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
asked Apr 15 '17 at 20:24
Vittorio Romeo
57.4k17154293
57.4k17154293
3
Specifically, the problem seems to be thatTsis not correctly deduced
– Ben Steffan
Apr 15 '17 at 20:50
add a comment |
3
Specifically, the problem seems to be thatTsis not correctly deduced
– Ben Steffan
Apr 15 '17 at 20:50
3
3
Specifically, the problem seems to be that
Ts is not correctly deduced– Ben Steffan
Apr 15 '17 at 20:50
Specifically, the problem seems to be that
Ts is not correctly deduced– Ben Steffan
Apr 15 '17 at 20:50
add a comment |
1 Answer
1
active
oldest
votes
To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH
I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
2
Consideringstd::tuplehas deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.
– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH
I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
2
Consideringstd::tuplehas deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.
– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
add a comment |
To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH
I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
2
Consideringstd::tuplehas deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.
– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
add a comment |
To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH
I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
To simplify your example further, it appears that GCC does not implement variadic template arguments in deduction guides:
https://wandbox.org/permlink/4YsacnW9wYcoceDH
I didn't see any explicit mention of variadic templates in the wording for deduction guides in the standard or on cppreference.com. I see no interpretation of the standard that disallows this. Therefore I think this is a bug.
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
edited Apr 16 '17 at 0:26
R Sahu
164k1291184
164k1291184
answered Apr 15 '17 at 20:48
Jackie
461211
answered Apr 15 '17 at 20:48
Jackie
461211
answered Apr 15 '17 at 20:48
Jackie
461211
461211
2
Consideringstd::tuplehas deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.
– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
add a comment |
2
Consideringstd::tuplehas deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.
– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
2
2
Considering
std::tuple has deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.– chris
Apr 15 '17 at 20:50
Considering
std::tuple has deduction guides, I'd say it's pretty safe to assume that variadic templates are allowed.– chris
Apr 15 '17 at 20:50
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
Agreed. Reported as bug #80438.
– Vittorio Romeo
Apr 15 '17 at 21:06
1
1
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
Yes, it's true, so I went looking for why this works. tuple uses a variadic deduction guide. However because of the enable_if in the tuple constructor the deduction guide doesn't match
– Jackie
Apr 15 '17 at 21:15
add a comment |
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3
Specifically, the problem seems to be that
Tsis not correctly deduced– Ben Steffan
Apr 15 '17 at 20:50