Can a variable be redeclared as auto that deduced to the same type?

Question

Is the following allowed by the standard?

#include <iostream>

extern int a;
auto a = 3;

int main(int, char**)
{
    std::cout << a << std::endl;
    return 0;
}

clang accepts the code. g++ complains for conflicting declaration.

Answer 1

Its not much clear to me from the standard, but then, there is this written

section 7.1.6.4 auto specifier
A program that uses auto in a context not explicitly allowed in this section is ill-formed.

Better read the mentioned section of the standard for all the allowed contexts.

Considering this, I believe g++ is correct and clang is wrong. But I could be wrong, there could be some separate section in standard which might be implying this context, but I could not find it.

Answer 2

Edit answer: As mention in the comments. The problem in this case is that writting

external int a;
auto a = 3;

is the same as writting

external int a;
int a = 3;

that means you have a new definition of a and that causes an error.

First answer: For my understanding this breaks parts of the One definition rule. Specifically, I mean the following rule (in reference to MISRA C++ 2008) that says that an identifier with external linkage should always have only one definition. In your example you have a definition in the current file(auto a = 3;) and with external you also refer to a definition in another file.