Constexpr if with a non-bool condition
I seem to have found something that clang and gcc disagree on. Here's the code:
int main() {
if constexpr (2) {}
}
This successfully compiles with gcc 7.4.0 but fails with clang 7.0.0 with this error message:
test.cpp:3:17: error: constexpr if condition evaluates to 2, which cannot be narrowed to type 'bool'
[-Wc++11-narrowing]
if constexpr (2) {}
^
1 error generated.
cppreference doesn't seem to mention "narrowing" so this seems like a clang bug but I'm not entirely certain. If this is a bug with either compiler, has it already been reported?
c++ language-lawyer c++17 implicit-conversion compiler-bug
asked 1 hour ago
Kerndog73Kerndog73
577724
add a comment |
I seem to have found something that clang and gcc disagree on. Here's the code:
int main() {
if constexpr (2) {}
}
This successfully compiles with gcc 7.4.0 but fails with clang 7.0.0 with this error message:
test.cpp:3:17: error: constexpr if condition evaluates to 2, which cannot be narrowed to type 'bool'
[-Wc++11-narrowing]
if constexpr (2) {}
^
1 error generated.
cppreference doesn't seem to mention "narrowing" so this seems like a clang bug but I'm not entirely certain. If this is a bug with either compiler, has it already been reported?
c++ language-lawyer c++17 implicit-conversion compiler-bug
asked 1 hour ago
Kerndog73Kerndog73
577724
What if you doif constexpr (!!2) {}?
– Jesper Juhl
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
I just checked.!!2works with clang
– Kerndog73
1 hour ago
add a comment |
I seem to have found something that clang and gcc disagree on. Here's the code:
int main() {
if constexpr (2) {}
}
This successfully compiles with gcc 7.4.0 but fails with clang 7.0.0 with this error message:
test.cpp:3:17: error: constexpr if condition evaluates to 2, which cannot be narrowed to type 'bool'
[-Wc++11-narrowing]
if constexpr (2) {}
^
1 error generated.
cppreference doesn't seem to mention "narrowing" so this seems like a clang bug but I'm not entirely certain. If this is a bug with either compiler, has it already been reported?
c++ language-lawyer c++17 implicit-conversion compiler-bug
asked 1 hour ago
Kerndog73Kerndog73
577724
I seem to have found something that clang and gcc disagree on. Here's the code:
int main() {
if constexpr (2) {}
}
This successfully compiles with gcc 7.4.0 but fails with clang 7.0.0 with this error message:
test.cpp:3:17: error: constexpr if condition evaluates to 2, which cannot be narrowed to type 'bool'
[-Wc++11-narrowing]
if constexpr (2) {}
^
1 error generated.
cppreference doesn't seem to mention "narrowing" so this seems like a clang bug but I'm not entirely certain. If this is a bug with either compiler, has it already been reported?
c++ language-lawyer c++17 implicit-conversion compiler-bug
c++ language-lawyer c++17 implicit-conversion compiler-bug
asked 1 hour ago
Kerndog73Kerndog73
577724
asked 1 hour ago
Kerndog73Kerndog73
577724
asked 1 hour ago
Kerndog73Kerndog73
577724
asked 1 hour ago
Kerndog73Kerndog73
577724
asked 1 hour ago
Kerndog73Kerndog73
577724
577724
What if you doif constexpr (!!2) {}?
– Jesper Juhl
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
I just checked.!!2works with clang
– Kerndog73
1 hour ago
add a comment |
What if you doif constexpr (!!2) {}?
– Jesper Juhl
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
I just checked.!!2works with clang
– Kerndog73
1 hour ago
What if you do
if constexpr (!!2) {} ?– Jesper Juhl
1 hour ago
What if you do
if constexpr (!!2) {} ?– Jesper Juhl
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
I just checked.
!!2 works with clang– Kerndog73
1 hour ago
I just checked.
!!2 works with clang– Kerndog73
1 hour ago
add a comment |
2 Answers
2
active
oldest
votes
Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.
[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 - And a proposal exists to change it, courtesy of Rakete1111 - https://wg21.link/p1401
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
add a comment |
We say it, but it's hidden. "contextually converted constant expression of type bool" is a standard term-of-art that excludes narrowing conversions.
Clang is correct.
answered 58 mins ago
T.C.T.C.
107k14220326
add a comment |
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.
[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 - And a proposal exists to change it, courtesy of Rakete1111 - https://wg21.link/p1401
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
add a comment |
Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.
[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 - And a proposal exists to change it, courtesy of Rakete1111 - https://wg21.link/p1401
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
add a comment |
Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.
[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 - And a proposal exists to change it, courtesy of Rakete1111 - https://wg21.link/p1401
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.
[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it's also quite reasonable to consider the fact a conversion to bool is usually intended to check for "truthiness", and so the narrowing nature of it shouldn't matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 - And a proposal exists to change it, courtesy of Rakete1111 - https://wg21.link/p1401
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
edited 48 secs ago
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
answered 58 mins ago
StoryTellerStoryTeller
99.9k12201271
99.9k12201271
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
add a comment |
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
A bug in the standard! LOL
– Kerndog73
56 mins ago
A bug in the standard! LOL
– Kerndog73
56 mins ago
1
1
There is a proposal for this, P1401
– Rakete1111
4 mins ago
There is a proposal for this, P1401
– Rakete1111
4 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
20 secs ago
add a comment |
We say it, but it's hidden. "contextually converted constant expression of type bool" is a standard term-of-art that excludes narrowing conversions.
Clang is correct.
answered 58 mins ago
T.C.T.C.
107k14220326
add a comment |
We say it, but it's hidden. "contextually converted constant expression of type bool" is a standard term-of-art that excludes narrowing conversions.
Clang is correct.
answered 58 mins ago
T.C.T.C.
107k14220326
add a comment |
We say it, but it's hidden. "contextually converted constant expression of type bool" is a standard term-of-art that excludes narrowing conversions.
Clang is correct.
answered 58 mins ago
T.C.T.C.
107k14220326
We say it, but it's hidden. "contextually converted constant expression of type bool" is a standard term-of-art that excludes narrowing conversions.
Clang is correct.
answered 58 mins ago
T.C.T.C.
107k14220326
answered 58 mins ago
T.C.T.C.
107k14220326
answered 58 mins ago
T.C.T.C.
107k14220326
answered 58 mins ago
T.C.T.C.
107k14220326
107k14220326
add a comment |
add a comment |
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What if you do
if constexpr (!!2) {}?– Jesper Juhl
1 hour ago
(!!2) will work, but 2 should work too.
– ivan.ukr
1 hour ago
I just checked.
!!2works with clang– Kerndog73
1 hour ago