Inconsistent behaviour across compilers in regard to instantiation of a template in a discarded if constexpr(false) statement
Clash Royale CLAN TAG#URR8PPP
I am trying to understand whether the snippet below should compile according to The Standard or not. When I try to compile it with latest version of three major compilers, the following occurs:
✓ Clang (version 7.0.0, with-std=c++17
flag): compiles fine;
✓ GCC (version 8.2, with-std=c++17
flag): also compiles fine;
❌ MSVC (version 19.16, with/std:c++17
flag): compiler error (see below).
The error occurs because the MSVC compiler seemingly tries to instantiate std::optional<void>
despite the fact that the code is discarded. GCC and Clang don't seem to do that.
Does The Standard clearly define what should occur in this case?
#include <optional>
#include <type_traits>
template<typename T, typename... Args>
struct Bar
void foo(Args... args)
if constexpr(!std::is_same_v<T, void>) // false
// MSVC compiler error occurs because of the line below; no error occurs when compiling with GCC and Clang
std::optional<T> val;
;
int main(int argc, char** argv)
Bar<void, int> inst;
inst.foo(1);
return 0;
Error by MSVC:
C:/msvc/v19_16/includeoptional(87): error C2182: '_Value': illegal use of type 'void'
C:/msvc/v19_16/includeoptional(128): note: see reference to class template instantiation 'std::_Optional_destruct_base<_Ty,false>' being compiled
with
[
_Ty=void
]
Live demo
c++ language-lawyer c++17
add a comment |
I am trying to understand whether the snippet below should compile according to The Standard or not. When I try to compile it with latest version of three major compilers, the following occurs:
✓ Clang (version 7.0.0, with-std=c++17
flag): compiles fine;
✓ GCC (version 8.2, with-std=c++17
flag): also compiles fine;
❌ MSVC (version 19.16, with/std:c++17
flag): compiler error (see below).
The error occurs because the MSVC compiler seemingly tries to instantiate std::optional<void>
despite the fact that the code is discarded. GCC and Clang don't seem to do that.
Does The Standard clearly define what should occur in this case?
#include <optional>
#include <type_traits>
template<typename T, typename... Args>
struct Bar
void foo(Args... args)
if constexpr(!std::is_same_v<T, void>) // false
// MSVC compiler error occurs because of the line below; no error occurs when compiling with GCC and Clang
std::optional<T> val;
;
int main(int argc, char** argv)
Bar<void, int> inst;
inst.foo(1);
return 0;
Error by MSVC:
C:/msvc/v19_16/includeoptional(87): error C2182: '_Value': illegal use of type 'void'
C:/msvc/v19_16/includeoptional(128): note: see reference to class template instantiation 'std::_Optional_destruct_base<_Ty,false>' being compiled
with
[
_Ty=void
]
Live demo
c++ language-lawyer c++17
2
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
For what it's worth: the motivation ofif constexpr
was more or less exactly to make examples like that compile.
– Barry
Jan 10 at 16:46
add a comment |
I am trying to understand whether the snippet below should compile according to The Standard or not. When I try to compile it with latest version of three major compilers, the following occurs:
✓ Clang (version 7.0.0, with-std=c++17
flag): compiles fine;
✓ GCC (version 8.2, with-std=c++17
flag): also compiles fine;
❌ MSVC (version 19.16, with/std:c++17
flag): compiler error (see below).
The error occurs because the MSVC compiler seemingly tries to instantiate std::optional<void>
despite the fact that the code is discarded. GCC and Clang don't seem to do that.
Does The Standard clearly define what should occur in this case?
#include <optional>
#include <type_traits>
template<typename T, typename... Args>
struct Bar
void foo(Args... args)
if constexpr(!std::is_same_v<T, void>) // false
// MSVC compiler error occurs because of the line below; no error occurs when compiling with GCC and Clang
std::optional<T> val;
;
int main(int argc, char** argv)
Bar<void, int> inst;
inst.foo(1);
return 0;
Error by MSVC:
C:/msvc/v19_16/includeoptional(87): error C2182: '_Value': illegal use of type 'void'
C:/msvc/v19_16/includeoptional(128): note: see reference to class template instantiation 'std::_Optional_destruct_base<_Ty,false>' being compiled
with
[
_Ty=void
]
Live demo
c++ language-lawyer c++17
I am trying to understand whether the snippet below should compile according to The Standard or not. When I try to compile it with latest version of three major compilers, the following occurs:
✓ Clang (version 7.0.0, with-std=c++17
flag): compiles fine;
✓ GCC (version 8.2, with-std=c++17
flag): also compiles fine;
❌ MSVC (version 19.16, with/std:c++17
flag): compiler error (see below).
The error occurs because the MSVC compiler seemingly tries to instantiate std::optional<void>
despite the fact that the code is discarded. GCC and Clang don't seem to do that.
Does The Standard clearly define what should occur in this case?
#include <optional>
#include <type_traits>
template<typename T, typename... Args>
struct Bar
void foo(Args... args)
if constexpr(!std::is_same_v<T, void>) // false
// MSVC compiler error occurs because of the line below; no error occurs when compiling with GCC and Clang
std::optional<T> val;
;
int main(int argc, char** argv)
Bar<void, int> inst;
inst.foo(1);
return 0;
Error by MSVC:
C:/msvc/v19_16/includeoptional(87): error C2182: '_Value': illegal use of type 'void'
C:/msvc/v19_16/includeoptional(128): note: see reference to class template instantiation 'std::_Optional_destruct_base<_Ty,false>' being compiled
with
[
_Ty=void
]
Live demo
c++ language-lawyer c++17
c++ language-lawyer c++17
edited Jan 10 at 12:27
YSC
21.6k350103
21.6k350103
asked Jan 10 at 10:45
NejcNejc
771212
771212
2
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
For what it's worth: the motivation ofif constexpr
was more or less exactly to make examples like that compile.
– Barry
Jan 10 at 16:46
add a comment |
2
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
For what it's worth: the motivation ofif constexpr
was more or less exactly to make examples like that compile.
– Barry
Jan 10 at 16:46
2
2
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
For what it's worth: the motivation of
if constexpr
was more or less exactly to make examples like that compile.– Barry
Jan 10 at 16:46
For what it's worth: the motivation of
if constexpr
was more or less exactly to make examples like that compile.– Barry
Jan 10 at 16:46
add a comment |
2 Answers
2
active
oldest
votes
Definitively a bug of MSVC. A bug report exist and has been reportedly fixed in Visual Studio 2019 Preview.
if constexpr
is standardized in [stmt.if]/2
:
If the
if
statement is of the formif constexpr
, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.
This applies.
If the value of the converted condition is false, the first substatement is a discarded statement, otherwise [...].
It also applies, making in your program std::optional<T> val;
a discarded statement.
During the instantiation of an enclosing templated entity (ndYSC
Bar<void, int>
), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
add a comment |
Along with @YSC's answer, also relevant is [temp.inst]/10
:
An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement , unless such instantiation is required.
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Definitively a bug of MSVC. A bug report exist and has been reportedly fixed in Visual Studio 2019 Preview.
if constexpr
is standardized in [stmt.if]/2
:
If the
if
statement is of the formif constexpr
, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.
This applies.
If the value of the converted condition is false, the first substatement is a discarded statement, otherwise [...].
It also applies, making in your program std::optional<T> val;
a discarded statement.
During the instantiation of an enclosing templated entity (ndYSC
Bar<void, int>
), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
add a comment |
Definitively a bug of MSVC. A bug report exist and has been reportedly fixed in Visual Studio 2019 Preview.
if constexpr
is standardized in [stmt.if]/2
:
If the
if
statement is of the formif constexpr
, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.
This applies.
If the value of the converted condition is false, the first substatement is a discarded statement, otherwise [...].
It also applies, making in your program std::optional<T> val;
a discarded statement.
During the instantiation of an enclosing templated entity (ndYSC
Bar<void, int>
), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
add a comment |
Definitively a bug of MSVC. A bug report exist and has been reportedly fixed in Visual Studio 2019 Preview.
if constexpr
is standardized in [stmt.if]/2
:
If the
if
statement is of the formif constexpr
, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.
This applies.
If the value of the converted condition is false, the first substatement is a discarded statement, otherwise [...].
It also applies, making in your program std::optional<T> val;
a discarded statement.
During the instantiation of an enclosing templated entity (ndYSC
Bar<void, int>
), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
Definitively a bug of MSVC. A bug report exist and has been reportedly fixed in Visual Studio 2019 Preview.
if constexpr
is standardized in [stmt.if]/2
:
If the
if
statement is of the formif constexpr
, the value of the condition shall be a contextually converted constant expression of type bool; this form is called a constexpr if statement.
This applies.
If the value of the converted condition is false, the first substatement is a discarded statement, otherwise [...].
It also applies, making in your program std::optional<T> val;
a discarded statement.
During the instantiation of an enclosing templated entity (ndYSC
Bar<void, int>
), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.
edited Jan 10 at 12:14
answered Jan 10 at 11:02
YSCYSC
21.6k350103
21.6k350103
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
add a comment |
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
I downloaded Visual Studio 2019 Preview and tried to compile the code with the compiler version 19.20 that shipped with it. The issue from the bug report which you mentioned is indeed solved ... but unfortunately my code snippet still does not compile (I get the same compiler errors as before).
– Nejc
Jan 11 at 16:13
1
1
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
I created a bug report: developercommunity.visualstudio.com/content/problem/425475/…
– Nejc
Jan 11 at 16:23
add a comment |
Along with @YSC's answer, also relevant is [temp.inst]/10
:
An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement , unless such instantiation is required.
add a comment |
Along with @YSC's answer, also relevant is [temp.inst]/10
:
An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement , unless such instantiation is required.
add a comment |
Along with @YSC's answer, also relevant is [temp.inst]/10
:
An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement , unless such instantiation is required.
Along with @YSC's answer, also relevant is [temp.inst]/10
:
An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement , unless such instantiation is required.
answered Jan 10 at 11:41
P.WP.W
12.9k3945
12.9k3945
add a comment |
add a comment |
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2
Looks like an MSVC bug, since IRC the part under constexpr if shouldn't be instantiated if it depends on a template parameter, unless the branch was taken.
– Dan M.
Jan 10 at 11:00
For what it's worth: the motivation of
if constexpr
was more or less exactly to make examples like that compile.– Barry
Jan 10 at 16:46