Clang accepts explicit instantiation of class-template member outside its namespace

using namespace A;

// Invalid: should be inside namespace A
template const int My_Class::S;
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.