Is it well-formed, if I redefine a variable as aut

Question

This question already has an answer here:

• Does a declaration using “auto” match an extern declaration that uses a concrete type specifier? 3 answers

Look at this snippet:

int a;
extern int b;
auto b = a;

Is it well-formed? Clang successfully compiles it, but GCC and MSVC don't.

(This issue has come up when I answered How to declare and define a static member with deduced type?)

Answer 1

Clang, GCC, MSVC. (This answer previous stated that all 3 compilers would refuse to build it, but that was incorrect.)

dcl.spec.auto does not address the compatibility of multiple declarations of the same variable when mixing the auto type specifier with other type specifiers. However, it addresses it for function return types:

auto f();
auto f() { return 42; } // return type is int
auto f();               // OK
int f();                // error, cannot be overloaded with auto f()
decltype(auto) f();     // error, auto and decltype(auto) don't match

So my intuition is that this is an oversight in the standard and the behavior is currently unspecified, but if/when it gets specified, there would be precedent to make it illegal. (On the other hand, variables can't be overloaded, so who knows.)

Answer 2

Tl;DR;

clang is correct, the logic is that this is allowed by [dcl.spec.auto] and to restrict this for deduced return types [dcl.spec.auto]p11 was added otherwise there is no restriction and therefore this is not restricted for the variables case.

See my more complete answer in the duplicate