[expr.const] Clarify wording of "usable in constant expressions"

[mpolacek]

Full name of submitter: Marek Polacek

Reference (section label): [expr.const]/4

Link to reflector thread (if any): N/A

Issue description: We say that V is usable in constant expressions if:

V is constexpr,
V is not initialized to a TU-local value, or
P is in the same translation unit as D.

but this is ambiguous; it could be read as either a || b || c or a && (b || c). The former is likely the intended reading.

Suggested resolution:

V is constexpr,<ins> or</ins>
V is not initialized to a TU-local value, or
P is in the same translation unit as D.

[t3nsor]

Why do you think that this grammatical structure can potentially be interpreted as "a && (b || c)"?

[mpolacek]

Why do you think that this grammatical structure can potentially be interpreted as "a && (b || c)"?

Because without the "or" or "and" it is not clear whether "V is constexpr" is a sufficient or necessary condition.

[t3nsor]

But we say things like "X, Y, and Z" and "X, Y, or Z" all the time. All native speakers understand these constructs to mean "X and Y and Z" and "X or Y or Z". So why is this part of the standard ambiguous?

[mpolacek]

But we say things like "X, Y, and Z" and "X, Y, or Z" all the time. All native speakers understand these constructs to mean "X and Y and Z" and "X or Y or Z". So why is this part of the standard ambiguous?

I guess I was confused because here it is immediately followed by "An object or reference is usable in constant expressions if it is" followed by a list where every bullet ends with "or". But, https://github.com/cplusplus/draft/wiki/Specification-Style-Guidelines#lists deems both styles OK so I suppose we can close this issue.