What precisely is an expression?

Question

Consider whether x in the declaration int x; is an expression.

I used to think that it's certainly not, but the grammar calls the variable name an id-expression here.

One could then argue that only expression is an expression, not ??-expression. But then in 1 + 2, neither 1 nor 2 match, because those are additive-expression and multiplicative-expression respectively, not expressions. But common sense says those should be called expressions too.

We could decide that any ??-expression (including expression) is an expression, but then the variable name in a declaration matches as well.

We could define an expression to be any ??-expression except id-expression, but this feels rather arbitrary.

What's the right grammatical definition of an expression, and is the variable name in its declaration an expression or not?

Answer 1

After looking at the links provided by @LanguageLawyer (1, 2), I'm convinced the consensus is that id-expression is a misnomer, and not always an expression (e.g. it's not an expression in a declaration).

Then, a source substring is an expression if at least one of its parents in the parse tree is called:

• expression, or
• *-expression¹ but not id-expression

, and that parent expands exactly to this substring and nothing more.

This is the same definition @n.m. proposed, except I allow "*-expression and not id-expression" nodes as well.

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¹ * is a wildcard for any string.