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 1 hour ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins 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 1 hour 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 1 hour ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins 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 1 hour ago
StoryTellerStoryTeller
99.9k12201271
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins 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 1 hour 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 1 hour ago
StoryTellerStoryTeller
99.9k12201271
edited 2 mins ago
answered 1 hour ago
StoryTellerStoryTeller
99.9k12201271
answered 1 hour ago
StoryTellerStoryTeller
99.9k12201271
answered 1 hour ago
StoryTellerStoryTeller
99.9k12201271
99.9k12201271
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins ago
add a comment |
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins ago
A bug in the standard! LOL
– Kerndog73
58 mins ago
A bug in the standard! LOL
– Kerndog73
58 mins ago
1
1
There is a proposal for this, P1401
– Rakete1111
6 mins ago
There is a proposal for this, P1401
– Rakete1111
6 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins ago
@Rakete1111 - shamelessly added to the answer :) Thank you!
– StoryTeller
2 mins 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 1 hour 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 1 hour 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 1 hour 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 1 hour ago
T.C.T.C.
107k14220326
answered 1 hour ago
T.C.T.C.
107k14220326
answered 1 hour ago
T.C.T.C.
107k14220326
answered 1 hour 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