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

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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.

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