规范有明确说明。这句：
“The type of such a data member is the referenced type if the entity is a reference to an object”。

ISO_IEC_14882__2020-12.pdf 这个在110页，"7.5.5.3 Captures"这一节的第10条：

10 An entity is captured by copy if
(10.1) — it is implicitly captured, the capture-default is =, and the captured entity is not *this, or
(10.2) — it is explicitly captured with a capture that is not of the form this, & identifier, or & identifier initializer.
For each entity captured by copy, an unnamed non-static data member is declared in the closure type. The
declaration order of these members is unspecified. The type of such a data member is the referenced type
if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity
is a reference to a function, or the type of the corresponding captured entity otherwise. A member of an
anonymous union shall not be captured by copy.
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FROM 221.218.165.*