[intro.execution] p8 if vs. if and only if for the definition

[xmh0511]

Full name of submitter (unless configured in github; will be published with the issue): Jim X

[intro.execution] p8 says:

An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

The if means this is a logical implication, which means that if we know X is sequenced before Y, we cannot conversely infer that every value computation and every side effect of X is sequenced before those of Y. Is this the intent?

The intent might be that they are equivalent from the wording

if A is sequenced before B (or, equivalently, B is sequenced after A), then the execution of A shall precede the execution of B.

Suggested Resolution:

An expression X is said to be sequenced before an expression Y if and only if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

[jensmaurer]

"is said to be" means this is introducing a definition (what "sequenced before" means when applied to "expression").

This pattern (with a simple "if") appears quite a bit in the standard, e.g. in [intro.races]p8.

[xmh0511]

@jensmaurer In certain places, we use "if and only if", for example, [temp.constr.order] p1:

A constraint P subsumes a constraint Q if and only if, for every disjunctive clause

This is also a definition of what constraint subsumes another, which obviously means it's a biconditional statement

Sometimes, we use "if" to only mean logical implication in the definition, for example, [class.default.ctor] p3

A default constructor for a class X is trivial if it is not user-provided and if

• [...]
• all the direct base classes of X have trivial default constructors, and

When the class X doesn't have a direct base class, it's a vacuous truth, which is only useful when it's a conditional statement.

We expect we can have a consistent expression here.