Refuse a mutable proxy_cast on a const contained value - #80
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proxy_cast_dispatch guarded a reference result with std::is_const_v<T>, where T is the deduced operand type and is therefore always a reference. A reference type is never const, so the guard never fired and a caller could ask a const proxy for a mutable reference to what it contains and get one, silently casting the constness away. The guard now tests the referenced type. proxy_cast<int&> on a proxy whose contained value is const throws bad_proxy_cast, and the pointer form returns nullptr, which is what the const-qualified overload already promised. Reaching a const contained value still works by asking for it as const.
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proxy_cast_dispatchto test the referenced type rather than the reference, so a const contained value is no longer reachable mutably.TestIndirectCast_Ref_ConstTargetandTestIndirectCast_Ptr_ConstTarget, covering the value form and the pointer form.proxy_cast<int&>on a proxy whose contained value is const now throwsbad_proxy_cast, and the pointer form returnsnullptr, which is what the const qualified overload already promised. Reaching a const contained value still works by asking for it as const.Resolves #79