Why does the following code result in a deadlock in g++ 5.4.0?
126.96.36.199 Mutex types [thread.mutex.requirements.mutex]
6 The expression m.lock() shall be well-formed and have the following
12 Throws: system_error when an exception is required (30.2.2).
13 Error conditions:
(13.1) — operation_not_permitted — if the thread does not have the
privilege to perform the operation.
(13.2) — resource_deadlock_would_occur — if the implementation detects
that a deadlock would occur.
(13.3) — device_or_resource_busy — if the mutex is already locked and
blocking is not possible.
Your code has undefined behaviour, because you are violating a precondition ([thread.mutex.requirements.mutex]):
m.lock()shall be well-formed and have the following semantics:
mis of type
std::mutex[...], the calling thread does not own the mutex.