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Consider this code:
struct A
{
template <typename T>
concept foo = true;
};
It doesn't compile. My Clang 10 gives me error: concept declarations may only appear in global or namespace scope, and GCC says something similar.
Is there a reason why it's not allowed? I don't see why it couldn't work, even if the enclosing class was a template.
601
Let's define a few concepts in this post. A concept can be defined by a function template or by a variable template. A variable template is new with C++14 and declares a family of variables. If you use a function template for your concept, it's called a function concept; in the second case a variable concept.
When you declare a program element such as a class, function, or variable, its name can only be "seen" and used in certain parts of your program. The context in which a name is visible is called its scope. For example, if you declare a variable x within a function, x is only visible within that function body.
It is possible to declare a class without defining it (forward declaration) as long as it is defined later on within the translation unit. In the case of functions, one can declare a function without defining it within the translation unit, and the linker will link it to its definition in a different translation unit.
The fundamental difficulty that would arise is that concepts could become dependent:
template<class T>
struct A {
template<T::Q X>
void f();
};
Is X a non-type template parameter of (dependent) type T::Q (which does not require typename in C++20), or is it a type template parameter constrained by the concept T::Q?
The rule is that it’s the former; we would need new syntax (along the lines of typename/template) to express the other possibility: perhaps something like
template<T::concept Q X> requires T::concept R<X*>
void A::g() {}
No one has explored such an extension seriously, and it could easily conflict with other extensions to concept syntax that might be more valuable.
136
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