Is it allowed to initialize array recursively?

Question

I have the following code snippet:

int i[] = {42, i[0]};

Is such initialization allowed or leads to undefined behaviour?

Three major compilers (gcc, clang, msvc) give me 42 for i[1]. Hence looks legit, but I would like to see a cite from the standard for this case.

Answer 1

Yes, it is well-defined.

int i[] = {42, i[0]};

This is an aggregate¹ initialization². Aggregate initialization observes this rule:

[dcl.init.aggr]/6

The initializations of the elements of the aggregate are evaluated in the element order. That is, all value computations and side effects associated with a given element are sequenced before those of any element that follows it in order.

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¹⁾ http://eel.is/c++draft/dcl.init.aggr#1

²⁾ http://eel.is/c++draft/dcl.init.aggr#3