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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.std.c
Subject: 7.1.4p2 vs. Annex J
Date: Mon, 03 Aug 2020 20:42:45 -0700
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Joe Zbiciak and I discovered this in a discussion on Quora.

N1570 7.1.4p2 says:

    Provided that a library function can be declared without reference
    to any type defined in a header, it is also permissible to declare
    the function and use it without including its associated header.

which means that this is a valid "Hello, world" program:

    int puts(const char *s);
    
    int main(void) {
        puts("Hello, world");
    }

But J.2 (which is non-normative) says that the behavior is undefined if:

    A function, object, type, or macro that is specified as being
    declared or defined by some standard header is used before any
    header that declares or defines it is included (7.1.2).

which the above program violates.

Is the intent behind that paragraph in J.2 something other than
what it seems to be?  (Or am I misreading it?)  Or is it just wrong?

Other than declaring a function, as permitted by 7.1.4p2, how would
a program even violate that paragraph in J.2 without also violating
a constraint?

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */