Constexpr if with a non-bool condition












6















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







share|improve this question























  • 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. !!2 works with clang

    – Kerndog73
    1 hour ago
















6















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







share|improve this question























  • 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. !!2 works with clang

    – Kerndog73
    1 hour ago














6












6








6








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







share|improve this question














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






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 1 hour ago









Kerndog73Kerndog73

572724




572724













  • 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. !!2 works with clang

    – Kerndog73
    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











  • I just checked. !!2 works 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












2 Answers
2






active

oldest

votes


















5














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 standard, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.






share|improve this answer


























  • A bug in the standard! LOL

    – Kerndog73
    55 mins ago






  • 1





    There is a proposal for this, P1401

    – Rakete1111
    3 mins ago



















3














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.






share|improve this answer























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    2 Answers
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    2 Answers
    2






    active

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    active

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    active

    oldest

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    5














    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 standard, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.






    share|improve this answer


























    • A bug in the standard! LOL

      – Kerndog73
      55 mins ago






    • 1





      There is a proposal for this, P1401

      – Rakete1111
      3 mins ago
















    5














    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 standard, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.






    share|improve this answer


























    • A bug in the standard! LOL

      – Kerndog73
      55 mins ago






    • 1





      There is a proposal for this, P1401

      – Rakete1111
      3 mins ago














    5












    5








    5







    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 standard, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.






    share|improve this answer















    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 standard, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 47 mins ago

























    answered 57 mins ago









    StoryTellerStoryTeller

    99.9k12201271




    99.9k12201271













    • A bug in the standard! LOL

      – Kerndog73
      55 mins ago






    • 1





      There is a proposal for this, P1401

      – Rakete1111
      3 mins ago



















    • A bug in the standard! LOL

      – Kerndog73
      55 mins ago






    • 1





      There is a proposal for this, P1401

      – Rakete1111
      3 mins ago

















    A bug in the standard! LOL

    – Kerndog73
    55 mins ago





    A bug in the standard! LOL

    – Kerndog73
    55 mins ago




    1




    1





    There is a proposal for this, P1401

    – Rakete1111
    3 mins ago





    There is a proposal for this, P1401

    – Rakete1111
    3 mins ago













    3














    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.






    share|improve this answer




























      3














      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.






      share|improve this answer


























        3












        3








        3







        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.






        share|improve this answer













        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.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 57 mins ago









        T.C.T.C.

        107k14220326




        107k14220326






























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