What does it mean when a behavior is identified as something that
'shall' or 'shall not' be done, but if it isn't or is:
1) the program is not ill-formed
2) the behavior isn't undefined
3) the behavior isn't implementation defined
4) no diagnostic is required

I'm thinking specifically of the requirement that user-defined names
'shall not' have double underscores in them, nor start with a single
underscore followed by a capital letter, but the situation also crops
up elsewhere.

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dabbler@cris.com (Dabbler@cris.com) writes:

>What does it mean when a behavior is identified as something that
>'shall' or 'shall not' be done, but if it isn't or is:
>1) the program is not ill-formed
>2) the behavior isn't undefined
>3) the behavior isn't implementation defined
>4) no diagnostic is required

If the "shall" specifies a requirement on the _behaviour_ of the
program, then if it is violated, the behaviour is undefined, and
no diagnostic is required.  However, if the "shall" specifies a
requirement on the _form_ of the program, then if it is violated,
the program is ill-formed, and a diagnostic is required.
See 1.3[intro.compliance]/5.

>I'm thinking specifically of the requirement that user-defined names
>'shall not' have double underscores in them, nor start with a single
>underscore followed by a capital letter, but the situation also crops
>up elsewhere.

That one is quite explicit, and doesn't use "shall":

 |   17.3.3.1  Reserved names                          [lib.reserved.names]
 |
 | 3 If the program declares or defines a name in a  context  where  it  is
 |   reserved,  other than as explicitly allowed by this clause, the behav-
 |   ior is undefined.

 |   17.3.3.1.2  Global names                            [lib.global.names]
 |
 |   1 Certain  sets  of names and function signatures are always reserved to
 |     the implementation:
 |
 |     --Each name that begins with an underscore  and  either  an  uppercase
 |       letter  or  another underscore (_lex.key_) is reserved to the imple-
 |       mentation for any use.

 |  17.3.3.1.3  External linkage                        [lib.extern.names]
 |
 | 3 Each name having two consecutive underscores (_lex.key_)  is  reserved
 |   to  the  implementation  for  use  as  a name with both extern "C" and
 |   extern "C++" linkage.

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