| Issue |
176922
|
| Summary |
Clang accepts explicit instantiation of class-template member outside its namespace
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
vrukesh
|
**What code / commands /steps will reproduce the problem?**
Compile the sample code:
`
namespace A {
template<class B>
struct My_Class {
template<class C>
static constexpr C S = C(1);
};
}
template<class B>
template<class C>
constexpr C A::My_Class<B>::S;
using namespace A;
// Invalid: should be inside namespace A
template const int My_Class<int>::S<int>;
int main() {}
`
**Compiler Explorer link:**
https://godbolt.org/z/8rdYf9dzd
**What is the expected result?**
The program is ill-formed; the compiler should diagnose the misplaced explicit instantiation.
As per the standard:
A class, function, variable, or member template specialization can be explicitly instantiated from its template.
A member function, member class or static data member of a class template can be explicitly instantiated
from the member definition associated with its class template. An explicit instantiation of a function template
or member function of a class template shall not use the inline or constexpr specifiers.
The syntax for explicit instantiation is:
explicit-instantiation:
externopt template declaration
There are two forms of explicit instantiation: an explicit instantiation definition and an explicit instantiation
declaration. An explicit instantiation declaration begins with the extern keyword.
An explicit instantiation “shall appear in an enclosing namespace of its template;” placing it at global scope is ill-formed. Compiler should issue a diagnostic or reject.
**What happens instead?**
Compilation is successful.
Invalid code is accepted; masks namespace mistakes and can lead to ODR issues.
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