Confusion about array initialization in C

Question

In C language, if initialize an array like this:

int a[5] = {1,2};

then all the elements of the array that are not initialized explicitly w

Answer 1

I think the C11 standard covers this behaviour and says that the result is unspecified, and I don't think C18 made any relevant changes in this area.

The standard language is not easy to parse. The relevant section of the standard is §6.7.9 Initialization. The syntax is documented as:

initializer:
                assignment-expression
                { initializer-list }
                { initializer-list , }
initializer-list:
                designationopt initializer
                initializer-list , designationopt initializer
designation:
                designator-list =
designator-list:
                designator
                designator-list designator
designator:
                [ constant-expression ]
                . identifier

Note that one of the terms is assignment-expression, and since a[2] = 1 is indubitably an assignment expression, it is allowed inside initializers for arrays with non-static duration:

§4 All the expressions in an initializer for an object that has static or thread storage duration shall be constant expressions or string literals.

One of the key paragraphs is:

§19 The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;¹⁵¹⁾ all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

¹⁵¹⁾ Any initializer for the subobject which is overridden and so not used to initialize that subobject might not be evaluated at all.

And another key paragraph is:

§23 The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified.¹⁵²⁾

¹⁵²⁾ In particular, the evaluation order need not be the same as the order of subobject initialization.

I'm fairly sure that paragraph §23 indicates that the notation in the question:

int a[5] = { a[2] = 1 };

leads to unspecified behaviour. The assignment to a[2] is a side-effect, and the evaluation order of the expressions are indeterminately sequenced with respect to one another. Consequently, I don't think there is a way to appeal to the standard and claim that a particular compiler is handling this correctly or incorrectly.