在§7.1.2 .4：

An inline function shall be defined in every translation unit in which
it is odr-used and shall have exactly the same definition in every
case (3.2). [ Note: A call to the inline function may be encountered
before its definition appears in the translation unit. — end note ] If
the definition of a function appears in a translation unit before its
first declaration as inline,the program is ill-formed. If a function
with external linkage is declared inline in one translation unit,it
shall be declared inline in all translation units in which it appears;
no diagnostic is required. An inline function with external linkage
shall have the same address in all translation units. A static local
variable in an extern inline function always refers to the same
object. A type defined within the body of an extern inline function is
the same type in every translation unit.

一些洞察这是什么和什么时候使用它？