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Constexpr if with a non-bool condition

Is a compiler forced to reject invalid constexpr?clang 3.3 and constexpr constraintsconstexpr bug in clang but not in gcc?Is function pointer comparison in a constexpr function allowed?When is std::string_view::operator== really constexpr?GCC and Clang disagree about C++17 constexpr lambda captures“if constexpr” interaction with “try in constexpr function” warningNested constexpr-if statement in discarded branch is still evaluated?std::variant modification in constexprC++ constexpr function in return statement

18

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 it 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

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What if you do if constexpr (!!2) {} ?
  
  – Jesper Juhl
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  (!!2) will work, but 2 should work too.
  
  – ivan.ukr
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I just checked. !!2 works with clang
  
  – Kerndog73
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  bugs.llvm.org/show_bug.cgi?id=39322 gcc.gnu.org/bugzilla/show_bug.cgi?id=87724#c1
  
  – Language Lawyer
  33 mins ago
  

  
  
  
  

add a comment | 

18

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 it 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

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What if you do if constexpr (!!2) {} ?
  
  – Jesper Juhl
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  (!!2) will work, but 2 should work too.
  
  – ivan.ukr
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I just checked. !!2 works with clang
  
  – Kerndog73
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  bugs.llvm.org/show_bug.cgi?id=39322 gcc.gnu.org/bugzilla/show_bug.cgi?id=87724#c1
  
  – Language Lawyer
  33 mins 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 it 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

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

int main() {

  if constexpr (2) {}

}

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.

share|improve this question

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

share|improve this question

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

edited 1 hour ago

Peter Mortensen

13.7k1986112

edited 1 hour ago

Peter Mortensen

13.7k1986112

asked 8 hours ago

Kerndog73Kerndog73

662825

asked 8 hours ago

Kerndog73Kerndog73

662825

2 Answers
2

active

oldest

votes

20

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}

share|improve this answer

edited 6 hours ago

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  A bug in the standard! LOL
  
  – Kerndog73
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  There is a proposal for this, P1401
  
  – Rakete1111
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @Rakete1111 - shamelessly added to the answer :) Thank you!
  
  – StoryTeller
  6 hours ago
  
  
  

  
  
  
  

add a comment | 

5

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

answered 7 hours ago

T.C.T.C.

107k14220326

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Did CWG agree that the current wording in the standard is intended?
  
  – Language Lawyer
  1 hour ago
  

  
  
  
  

add a comment | 

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20

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}

share|improve this answer

edited 6 hours ago

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  A bug in the standard! LOL
  
  – Kerndog73
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  There is a proposal for this, P1401
  
  – Rakete1111
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @Rakete1111 - shamelessly added to the answer :) Thank you!
  
  – StoryTeller
  6 hours ago
  
  
  

  
  
  
  

add a comment | 

20

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}

share|improve this answer

edited 6 hours ago

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  A bug in the standard! LOL
  
  – Kerndog73
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  There is a proposal for this, P1401
  
  – Rakete1111
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @Rakete1111 - shamelessly added to the answer :) Thank you!
  
  – StoryTeller
  6 hours 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}

share|improve this answer

edited 6 hours ago

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

share|improve this answer

edited 6 hours ago

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

answered 7 hours ago

StoryTellerStoryTeller

100k12202272

5

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

answered 7 hours ago

T.C.T.C.

107k14220326

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Did CWG agree that the current wording in the standard is intended?
  
  – Language Lawyer
  1 hour ago
  

  
  
  
  

add a comment | 

5

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

answered 7 hours ago

T.C.T.C.

107k14220326

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Did CWG agree that the current wording in the standard is intended?
  
  – Language Lawyer
  1 hour 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.

share|improve this answer

answered 7 hours ago

T.C.T.C.

107k14220326

share|improve this answer

answered 7 hours ago

T.C.T.C.

107k14220326

answered 7 hours ago

T.C.T.C.

107k14220326

answered 7 hours ago

T.C.T.C.

107k14220326