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Nothing is preventing the code from returning both at the same time. I've seen code (including in the standard library) that returns both an error and a return value. In a language with disjoint unions, such cases would be encoded properly.

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convention prevents it

and in the case where returning both is OK, then documentation makes that clear

this is not difficult

Convention in no way prevents anything. Convention is simply that. People are free to not follow convention when nothing is enforcing it. You frequently see juniors, who may be brand new to the language, making mistakes with conventions. If I'm supposed to depend on the vagaries of some accepted standard that is only documented in text then it is less than useless in the real world

if we say a language "addresses" a given concern, is it necessary that this is accomplished in the compiler, and that the rules for that concern, whatever they are, are enforced at compile-time?

(spoiler: no)

spoiler: yes

If the language "addresses" it by convention then it is not addressing it at a language level at all

"the language level" is not only what is defined and enforced by the compiler

but i'm sure i won't convince you of anything here, so good luck to you

We all know that documentation always 100% matches the behavior of the code :-)

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