Does a declaration using “auto” match an extern declaration that uses a concrete type specifier?

Question

Consider the following program:

extern int x;
auto x = 42;
int main() { }

Clang 3.5 accepts it (live demo), GCC 4.9 and VS2013 do not (live

Answer 1

I'd imagine the restriction in [dcl.spec.auto]p11 exists because otherwise, that would allow:

int f();
auto f(); // What's the return type here?

The thing is, you can have an undeduced type type has the return type of a function. There are no deduction rules based on previous declarations, which is why such mixing is disallowed for functions, even though the following would be perfectly fine:

int f();
auto f() { return 1; }

This problem does not exist for variables:

extern int v;
extern auto v; // ill-formed

Any declaration-only variables has to use a non-placeholder type. What this means is that if you use a placeholder type for the definition of v, it can get deduced without any problems and then of course has to match the non-placeholder type used in the first declaration.

extern int v;
auto v = 1; // ok, type deduced as 'int', matches first declaration.