c++ - Questions about the strange rules of lambda default = capture - Stack Overflow

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I see the following rules in eel and cppref

Within the body of a lambda with capture default =, the type of any capturable entity is as if it were captured (and thus const-qualification is often added if the lambda is not mutable), even though the entity is in an unevaluated operand and not captured (e.g. in decltype)”

If a nested lambda m2 captures something that is also captured by the immediately enclosing lambda m1, then m2's capture is transformed as follows:

• if the enclosing lambda m1 captures by copy, m2 is capturing the non-static member of m1's closure type, not the original variable or *this; if m1 is not mutable, the non-static data member is considered to be const-qualified.

• if the enclosing lambda m1 captures by reference, m2 is capturing the original variable or *this.

Now I'm wondering, is the const qualification they describe for value define type decltype(id) or value expr type decltype((id))?

And what is the concept of "often" for the first rule?

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