Why does aggregate initialization not work anymore since C++20 if a constructor is explicitly defaulted or deleted?

The abstract from P1008, the proposal that led to the change:

C++ currently allows some types with user-declared constructors to be initialized via aggregate initialization, bypassing those constructors. The result is code that is surprising, confusing, and buggy. This paper proposes a fix that makes initialization semantics in C++ safer, more uniform,and easier to teach. We also discuss the breaking changes that this fix introduces.

One of the examples they give is the following.

struct X {
  int i{4};
  X() = default;
};

int main() {
  X x1(3); // ill-formed - no matching c’tor
  X x2{3}; // compiles!
}

To me, it's quite clear that the proposed changes are worth the backwards-incompatibility they bear. And indeed, it doesn't seem to be good practice anymore to = default aggregate default constructors.

The reasoning from P1008 (PDF) can be best understood from two directions:

1. If you sat a relatively new C++ programmer down in front of a class definition and ask "is this an aggregate", would they be correct?

The common conception of an aggregate is "a class with no constructors". If Typename() = default; is in a class definition, most people will see that as having a constructor. It will behave like the standard default constructor, but the type still has one. That is the broad conception of the idea from many users.

An aggregate is supposed to be a class of pure data, able to have any member assume any value it is given. From that perspective, you have no business giving it constructors of any kind, even if you defaulted them. Which brings us to the next reasoning:

2. If my class fulfills the requirements of an aggregate, but I don't want it to be an aggregate, how do I do that?

The most obvious answer would be to = default the default constructor, because I'm probably someone from group #1. Obviously, that doesn't work.

Pre-C++20, your options are to give the class some other constructor or to implement one of the special member functions. Neither of these options are palatable, because (by definition) it's not something you actually need to implement; you're just doing it to make some side effect happen.

Post-C++20, the obvious answer works.

By changing the rules in such a way, it makes the difference between an aggregate and non-aggregate visible. Aggregates have no constructors; so if you want a type to be an aggregate, you don't give it constructors.

Oh, and here's a fun fact: pre-C++20, this is an aggregate:

class Agg
{
  Agg() = default;
};

Note that the defaulted constructor is private, so only people with private access to Agg can call it... unless they use Agg{}, bypasses the constructor and is perfectly legal.

The clear intent of this class is to create a class which can be copied around, but can only get its initial construction from those with private access. This allows forwarding of access controls, as only code which was given an Agg can call functions that take Agg as a parameter. And only code with access to Agg can create one.

Or at least, that's how it is supposed to be.

Now you could fix this more targetedly by saying that it's an aggregate if the defaulted/deleted constructors are not publicly declared. But that feels even more in-congruent; sometimes, a class with a visibly declared constructor is an aggregate and sometimes it isn't, depending on where that visibly declared constructor is.

Actually, MSDN addressed your concern in the below document:

Modified specification of aggregate type

In Visual Studio 2019, under /std:c++latest, a class with any user-declared constructor (for example, including a constructor declared = default or = delete) isn't an aggregate. Previously, only user-provided constructors would disqualify a class from being an aggregate. This change puts additional restrictions on how such types can be initialized.