Can I legally reinterpret_cast between layout-compatible standard-layout types?

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but I can’t see anything in the standard that therefore allows me to reinterpret_cast between them, even though that seems like the reasonable interpretation of “value representation”. Is this technically allowed by the standard?

No. The standard is clear (see [basic.lval] p10) about which types can be aliased, and layout-compatible types are not included.

If not, what does knowing the value representation of a type give you?

If the types are both trivially copyable and have the same value representation then you could memcpy from an object of one type to an object of the other type, and vice versa. If they’re not trivially copyable then it doesn’t give you much at all.

AFAICT the standard doesn’t actually say what can and can’t be done with layout-compatible types.

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