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Message: <blockquote data-quote="James Kuyper" data-source="post: 5149395">As a practical matter (I'll get to issue of what the standard saysfarther down), same size is insufficient to enable two types to becompatible. They must also have the same representation and alignmentrequirements, and must use the same mechanism when passed as a parameterand when returned as the value of a function.I've argued in the past that the standard guarantees compatibility forcertain pairs of types (which do not include "int" and "long"), but thatit does not prohibit other pairs of types from happening to becompatible on particular implementations. I was told that the caseswhere the standard specifies that two types are compatible areexhaustive - no other pairs of types can be compatible. This is anexample of a more general rule when interpreting the standard: wheneverit provides a list of things for which something is true, that list isexhaustive unless the standard explicitly says otherwise. I knew of thatrule, but find that argument less than compelling, because thecompatibility rules are not provided as a list, but as severalindependent clauses in widely separated parts of the standard.However, the official interpretation is that whenever a constraintrequires that two types be compatible, it is a violation of thatconstraint, requiring a diagnostic, if the two types are not onesexplicitly specified as being compatible by the standard.After issuing the required diagnostic, the implementation is free toproduce an executable anyway, and if you chose to execute the resultingprogram it might behave in exactly the same fashion as would bemandatory if the standard had specified that those two types werecompatible, and the implementation is free to document this as a fact.However, the diagnostic message is still the only mandatory result oftranslating such a program. The implementation's documentation cannot,therefore, accurately describe this fact by saying "int and long int arecompatible"; it must use some other wording to describe that fact.</blockquote>

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