attribute_type and attribute_const_type when type is deduced can be
different than when the type is provided.
Indeed when specifying attribute_type and attribute_const_type manually
it's possible to provide a type which isn't const qualified as
attribute_const_type. When deducing the types from the get_expr, a const
and a non const qualified type is taken respectively for attribute_type
and attribute_const_type.
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