Is `template<typename>;` legal C++?

Question

GCC and Clang disagree on whether template<typename>; is a valid statement in C++ at global scope.

I'd expect it not to be allowed in the C++ standard because templatization pertains to declaration statements, not to expression statements and in consequence also not to null statements (the statement ;).

So, is this a bug in Clang?

Answer 1

It's clang's idiosyncratic behavior that has existed for long time: missing declarations generate only a warning. It's just same as this:

int;

g++ would show an error, while clang will only show a warning. This doesn't contradict the standard.

warning: declaration does not declare anything [-Wmissing-declarations]

-Werror=missing-declarations sets things straight.

Answer 2

Not really. The standard explicitly disallows such a declaration in [temp]p2;

The declaration in a template-declaration (if any) shall

• declare or define a function, a class, or a variable, or

• define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or

• define a member template of a class or class template, or

• be a deduction-guide, or

• be an alias-declaration.

An empty-declaration doesn't match any of those clauses. Now the standard says that an implementation is required to issue a diagnostic message for any violation of its rules, like this one. Note that it says diagnostic, it doesn't specify whether a warning or an error (or even a note) is issued. The compiler can provide extensions that make what you wrote valid, as it wouldn't change the meaning of a well-formed program.

So no, both are right. However, clang's behavior is due to an extension, not something that the standard specifies.