which is the full-expression when the rule says the full-expression of initialization

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struct S {
  constexpr S(int i): I(i),D(i) { }       // full-expressions are initialization of I and initialization of D 
private:
  int I;
  int D;
};
int main(){
  constexpr S s1 = 1;  //full-expression comprises call of S​::​S(int)
}

According to the definition of full-expression:

A full-expression is

  1. an unevaluated operand,
  2. a constant-expression,
  3. an init-declarator or a mem-initializer, including the constituent expressions of the initializer,
  4. an invocation of a destructor generated at the end of the lifetime of an object other than a temporary object, or
  5. an expression that is not a subexpression of another expression and that is not otherwise part of a full-expression.

For an initializer, performing the initialization of the entity (including evaluating default member initializers of an aggregate) is also considered part of the full-expression.

The bullet 3 says s1 = 1 is a full-expression because it's an init-declarator and I(i) is a full-expression due to it's a mem-initializer and similarly for D(i). It means that initialize entity s1 can contain more than one full-expression? In this case, Which is the full-expression of the initialization in this set of full-expressions?

c++17 language-lawyer c++
1 Answers

Of course full-expressions can be dynamically nested: consider

void f(int i) {
  ++i;   // (useless) full-expression
}
void g() {
  f(1);  // full-expression
}

As such, there’s no conflict between initializing s1 being part of the init-declarator full-expression while also containing full-expressions for its mem-initializers.

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