Actually sounds like it isn't mandated by the standard after all; it's mandated for block thread_locals but not for thread_locals in the global scope:
3.7.2/2 [basic.stc.thread]: A variable with thread storage duration shall be initialized before its first odr-use (3.2) and, if constructed, shall be destroyed on thread exit.
This allows the constructor to be called at any point before the first use, similarly to "normal" globals, though implementations made different tradeoffs in these 2 cases
3.7.2/2 [basic.stc.thread]: A variable with thread storage duration shall be initialized before its first odr-use (3.2) and, if constructed, shall be destroyed on thread exit.
This allows the constructor to be called at any point before the first use, similarly to "normal" globals, though implementations made different tradeoffs in these 2 cases