Of course the members of a constant value are const too! A constant value can contain a pointer to or slice of non-const data though, which is why you can append to m.items[0].o. Maybe the language reference should explicitly state that all values, not just parameters, are copied by-value. This seems to be generally implicitly understood for systems languages, but I guess it can be surprising if you are coming from an object-oriented language.
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Of course the members of a constant value are const too! A constant value can contain a pointer to or slice of non-const data though, which is why you can append to m.items[0].o. Maybe the language reference should explicitly state that all values, not just parameters, are copied by-value. This seems to be generally implicitly understood for systems languages, but I guess it can be surprising if you are coming from an object-oriented language.