[temp.pre] requires-clause grammar doesn't allow for use of operator with template id

[ranaanoop]

Full name of submitter: Anoop Rana

Reference (section label): [temp.pre]

Link to reflector thread (if any): https://stackoverflow.com/questions/78155107/requires-compile-with-msvc-but-rejected-by-gcc

Issue description:

The !std::is_enum_v<T> without the parentheses in the below program is ill-formed as per the grammar rules. We see implementation divergence.

#include <type_traits>

template<typename T> requires !std::is_enum_v<T>  //invalid per grammar rules
void foo(T)
{
}

template<typename T> requires std::is_enum_v<T>  //valid per grammar rules
void bar(T)
{
} 

I don't see any reason why !std::is_enum_v<T> is disallowed but std::is_enum_v<T> is allowed. Note that all compilers accept the program if we remove the ! from std::is_enum_v<T>. Demo. That is, I think !std::is_enum_v<T> should also be allowed.

[t3nsor]

Isn't this a feature request?

[ranaanoop]

Isn't this a feature request?

Looks more like an oversight to me. I can write a proposal to EWG etc if thats where this should be submitted.

[jensmaurer]

I don't think there is a question what the standard says in this situation; the grammar in [temp.pre] is pretty obvious.

Also, the grammar as we have it correctly maps to the decomposition yielding atomic constraints. For example, !x is an atomic constraint by itself.

From a design perspective, negated constraints should be rare (and are expensive to implement), so I'm not seeing an urgent need for change.

Extension requests go to EWG via paper, please.

[AlanVoore]

Looking at this comment, it seems that it does make sense to allow omitting parentheses for other operators also.

Note c++ allows omitting parentheses for || and && but not for any other operator.

[jensmaurer]

Note c++ allows omitting parentheses for || and && but not for any other operator.

Yes, because stuff connected via || and && is dissected into atomic constraints. Not so for any other operator.

Any changes in this area need a paper to EWG with rationale and analysis of the resulting grammar ambiguities, in particular since contracts are arriving soon.