Order of definition of constrainsts and concepts c 20-CodePudding

Where is the right position of a concept or constraint?

The following code compile:

void f(int x) { } 

template <typename T>
concept Concept =
    requires (T a)
    { f(a); };


template <typename T>
    requires Concept<T>
struct A
{ };



int main(){
    A<int> a;
}

but if I change the position of the function f(int) to:

template <typename T>
concept Concept =
    requires (T a)
    { f(a); };


void f(int x) { } // <-- new position

template <typename T>
    requires Concept<T>
struct A
{ };



int main(){
    A<int> a;
}

this doesn't compile in gcc 11.3.0. I get the following error:

main.cpp:27:10: error: template constraint failure for ‘template<class T>  requires  Concept<T> struct A’
   27 |     A<int> a;
      |          ^
main.cpp:27:10: note: constraints not satisfied
main.cpp: In substitution of ‘template<class T>  requires  Concept<T> struct A [with T = int]’:
main.cpp:27:10:   required from here
main.cpp:12:9:   required for the satisfaction of ‘Concept<T>’ [with T = int]
main.cpp:13:5:   in requirements with ‘T a’ [with T = int]
main.cpp:14:8: note: the required expression ‘f(a)’ is invalid
   14 |     { f(a); };

CodePudding user response：

If a function is declared after a template, the only way the template can call it is via ADL. int is a built-in type, so ADL doesn't happen for it.

If the parameter was e.g. a class type, defined in the same namespace as f(), it would work.

Clang explains this:

...
<source>:5:7: note: because 'f(a)' would be invalid: call to function 'f' that is neither visible in the template definition nor found by argument-dependent lookup

CodePudding user response：

It works if you predeclare the function. It is not defined at that point in the file.

It's the same principle as when calling functions, you need a predeclaration to be able to use it without seeing the definition.

void f(int x);

template <typename T>
concept Concept =
    requires (T a)
    { f(a); };


void f(int x) { } // <-- new position

template <typename T>
    requires Concept<T>
struct A
{ };



int main(){
    A<int> a;
}