(From submission #460.)

Consider:

  struct B {
    constexpr ~B(){}      // #1
  };
  struct A {
    constexpr A(const B&){}  // #2
    constexpr ~A(){}
  };
  constexpr A a(B{});  // #3, checks #1 and #2

Subclause 7.7 [expr.const] bullet 6.1 specifies:

A variable v is constant-initializable if

• the full-expression of its initialization is a constant expression when interpreted as a constant-expression...
• ...

However, a full-expression need not satisfy the syntax of a constant-expression, and indeed, for the example at #3, the only syntactical expression is B{}, but the checking of the constructor of A and the destructor of B is intended to be part of the conditions for constant-initializable, which unsuccessfully attempts to delegate to 7.7 [expr.const] paragraph 22.