A `mtx_t` object has a lock state that affects whether a thread can proceed, even though that state is not an ordinary value the program reads. A `once_flag` has initialization state, and an `atomic_flag` can be set or cleared. These are forms of hidden state: parts of an execution’s abstract state alongside points of execution, intermediate values, and stored values. A change to hidden state is an effect. section 21.0
Ordinary stored values matter too. If a function receives pointers, a call may change an object through either pointer; when the pointers alias, an assignment through one can change a later read through the other. Reasoning about state therefore requires asking what can change, not just inspecting the local expression. The full abstract state may be difficult to determine and, with concurrent threads, may not have a single fine-grained moment at which to inspect it. section 12.3 section 21.0
#pragma FP_CONTRACT OFF #pragma FENV_ROUND FE_TONEAREST
Floating-point settings are a less visible dependency. These pragmas select contraction and rounding behavior; a calculation that appears to depend only on its arguments may also depend on the floating-point environment. section 16.3
TAKEAWAY: Check implicit state before treating a function as pure. A function marked `[[unsequenced]]` must not read nonconstant global variables or system state, or visibly modify them. Floating-point arithmetic generally prevents that designation because its behavior can depend on state such as the rounding mode. section 16.3
