Can a concept evaluation depend on where it is evaluated?
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14
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[temp.concept]/5 says:
A concept is not instantiated ([temp.spec]).
[ Note: An id-expression that denotes a concept specialization is evaluated as an expression ([expr.prim.id]). [...]]
Does it mean that this rule bellow ([temp.point]/8) does not apply?
If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.
For example if this rule does not apply, this code bellow is well formed:
template<class T>
concept Complete = sizeof(T)==sizeof(T);
struct A;
constexpr inline bool b1 = Complete<A>; //Complete<A>==false;
struct A{};
constexpr inline bool b2 = Complete<A>; //Complete<A>==true;
This question is followed by this one
c++ language-lawyer c++-concepts c++20
asked Nov 12 at 10:21
Oliv
8,1351954
|
show 2 more comments
up vote
14
down vote
favorite
[temp.concept]/5 says:
A concept is not instantiated ([temp.spec]).
[ Note: An id-expression that denotes a concept specialization is evaluated as an expression ([expr.prim.id]). [...]]
Does it mean that this rule bellow ([temp.point]/8) does not apply?
If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.
For example if this rule does not apply, this code bellow is well formed:
template<class T>
concept Complete = sizeof(T)==sizeof(T);
struct A;
constexpr inline bool b1 = Complete<A>; //Complete<A>==false;
struct A{};
constexpr inline bool b2 = Complete<A>; //Complete<A>==true;
This question is followed by this one
c++ language-lawyer c++-concepts c++20
asked Nov 12 at 10:21
Oliv
8,1351954
@bolov Ortemplate<class A>:-P
– Zereges
Nov 12 at 10:59
1
Are you sure the concept definition is well-formed when applied forb1?
– rubenvb
Nov 12 at 11:03
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - Yes,sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".
– StoryTeller
Nov 12 at 13:01
|
show 2 more comments
up vote
14
down vote
favorite
up vote
14
down vote
favorite
[temp.concept]/5 says:
A concept is not instantiated ([temp.spec]).
[ Note: An id-expression that denotes a concept specialization is evaluated as an expression ([expr.prim.id]). [...]]
Does it mean that this rule bellow ([temp.point]/8) does not apply?
If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.
For example if this rule does not apply, this code bellow is well formed:
template<class T>
concept Complete = sizeof(T)==sizeof(T);
struct A;
constexpr inline bool b1 = Complete<A>; //Complete<A>==false;
struct A{};
constexpr inline bool b2 = Complete<A>; //Complete<A>==true;
This question is followed by this one
c++ language-lawyer c++-concepts c++20
asked Nov 12 at 10:21
Oliv
8,1351954
[temp.concept]/5 says:
A concept is not instantiated ([temp.spec]).
[ Note: An id-expression that denotes a concept specialization is evaluated as an expression ([expr.prim.id]). [...]]
Does it mean that this rule bellow ([temp.point]/8) does not apply?
If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.
For example if this rule does not apply, this code bellow is well formed:
template<class T>
concept Complete = sizeof(T)==sizeof(T);
struct A;
constexpr inline bool b1 = Complete<A>; //Complete<A>==false;
struct A{};
constexpr inline bool b2 = Complete<A>; //Complete<A>==true;
This question is followed by this one
c++ language-lawyer c++-concepts c++20
c++ language-lawyer c++-concepts c++20
asked Nov 12 at 10:21
Oliv
8,1351954
asked Nov 12 at 10:21
Oliv
8,1351954
edited Nov 12 at 13:35
asked Nov 12 at 10:21
Oliv
8,1351954
asked Nov 12 at 10:21
Oliv
8,1351954
asked Nov 12 at 10:21
Oliv
8,1351954
8,1351954
@bolov Ortemplate<class A>:-P
– Zereges
Nov 12 at 10:59
1
Are you sure the concept definition is well-formed when applied forb1?
– rubenvb
Nov 12 at 11:03
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - Yes,sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".
– StoryTeller
Nov 12 at 13:01
|
show 2 more comments
@bolov Ortemplate<class A>:-P
– Zereges
Nov 12 at 10:59
1
Are you sure the concept definition is well-formed when applied forb1?
– rubenvb
Nov 12 at 11:03
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - Yes,sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".
– StoryTeller
Nov 12 at 13:01
@bolov Or
template<class A> :-P– Zereges
Nov 12 at 10:59
@bolov Or
template<class A> :-P– Zereges
Nov 12 at 10:59
1
1
Are you sure the concept definition is well-formed when applied for
b1?– rubenvb
Nov 12 at 11:03
Are you sure the concept definition is well-formed when applied for
b1?– rubenvb
Nov 12 at 11:03
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - Yes,
sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".– StoryTeller
Nov 12 at 13:01
@Oliv - Yes,
sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".– StoryTeller
Nov 12 at 13:01
|
show 2 more comments
1 Answer
1
active
oldest
votes
up vote
7
down vote
accepted
Can a concept evaluation depend on where it is evaluated?
Yes. This was explicitly discussed during core wording review when merging Concepts into the working draft. The concept is re-evaluated every time.
As a result, this:
template<class T>
concept Complete = sizeof(T) == sizeof(T);
struct A;
static_assert(!Complete<A>);
struct A {};
static_assert(Complete<A>);
is well-formed. In other words, we don't "memoize" concepts in the same way we "memoize" template instantiations.
answered Nov 12 at 18:43
Barry
176k18301557
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
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
up vote
7
down vote
accepted
Can a concept evaluation depend on where it is evaluated?
Yes. This was explicitly discussed during core wording review when merging Concepts into the working draft. The concept is re-evaluated every time.
As a result, this:
template<class T>
concept Complete = sizeof(T) == sizeof(T);
struct A;
static_assert(!Complete<A>);
struct A {};
static_assert(Complete<A>);
is well-formed. In other words, we don't "memoize" concepts in the same way we "memoize" template instantiations.
answered Nov 12 at 18:43
Barry
176k18301557
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
add a comment |
up vote
7
down vote
accepted
Can a concept evaluation depend on where it is evaluated?
Yes. This was explicitly discussed during core wording review when merging Concepts into the working draft. The concept is re-evaluated every time.
As a result, this:
template<class T>
concept Complete = sizeof(T) == sizeof(T);
struct A;
static_assert(!Complete<A>);
struct A {};
static_assert(Complete<A>);
is well-formed. In other words, we don't "memoize" concepts in the same way we "memoize" template instantiations.
answered Nov 12 at 18:43
Barry
176k18301557
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
add a comment |
up vote
7
down vote
accepted
up vote
7
down vote
accepted
Can a concept evaluation depend on where it is evaluated?
Yes. This was explicitly discussed during core wording review when merging Concepts into the working draft. The concept is re-evaluated every time.
As a result, this:
template<class T>
concept Complete = sizeof(T) == sizeof(T);
struct A;
static_assert(!Complete<A>);
struct A {};
static_assert(Complete<A>);
is well-formed. In other words, we don't "memoize" concepts in the same way we "memoize" template instantiations.
answered Nov 12 at 18:43
Barry
176k18301557
Can a concept evaluation depend on where it is evaluated?
Yes. This was explicitly discussed during core wording review when merging Concepts into the working draft. The concept is re-evaluated every time.
As a result, this:
template<class T>
concept Complete = sizeof(T) == sizeof(T);
struct A;
static_assert(!Complete<A>);
struct A {};
static_assert(Complete<A>);
is well-formed. In other words, we don't "memoize" concepts in the same way we "memoize" template instantiations.
answered Nov 12 at 18:43
Barry
176k18301557
answered Nov 12 at 18:43
Barry
176k18301557
answered Nov 12 at 18:43
Barry
176k18301557
answered Nov 12 at 18:43
Barry
176k18301557
176k18301557
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
add a comment |
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
Thanks! Could you have a look at this question too?
– Oliv
Nov 12 at 18:50
1
1
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
Note that it’s ill-formed NDR to have a template whose instantiations might be different because of such changes between two of its (potential) points of instantiation. (Mostly this affects function templates.)
– Davis Herring
Nov 12 at 19:40
add a comment |
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@bolov Or
template<class A>:-P– Zereges
Nov 12 at 10:59
1
Are you sure the concept definition is well-formed when applied for
b1?– rubenvb
Nov 12 at 11:03
That concept is ill-formed upon definition IIUC.
– StoryTeller
Nov 12 at 11:05
@Oliv - The paragraph I linked says nothing about instantiation, it's about template definition and name lookup. I'd be shocked if one can conjure non-dependent identifiers willy-nilly in a concept.
– StoryTeller
Nov 12 at 12:59
@Oliv - Yes,
sizeof(A). It was an entirely different question with that typo. But whatever, if it was "obvious".– StoryTeller
Nov 12 at 13:01