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I might be wrong here, but I think that the first one is a declaration of a guard with the name m using the default constructor.

The same way that int (a); and int a; are equivalent.

So the guard should remain until end of scope, but won't guard anything.

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Lock guard has no default constructor, so the line will in fact materialize a temporary lock guard around the mutex m, then throw it away at the semi-colon, thus locking nothing.

lock_guard doesn’t have a default ctor, you have to pass a mutex (or presumably invoke a move ctor, but I can’t see one on cppref).

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