Is it correct such an initialization of a multi-dimensional array in C++ 20 using an expression list?
int a[][3]( 1, 2, 3, { 4, 5, 6 } );
At least the C++ compiler of MS VS 2019 does not compile such a declaration
This declaration
int a[][3]( 1, 2, 3, 4, 5, 6 );
is not also compiled while this declaration
int a[][3]( { 1, 2, 3 }, { 4, 5, 6 } );
is compiled successfuly.
In the corresponding quote from the C++ 20 Standard (9.4 Initializers) relative to array initialization there is nothing said about whether brace elision is allowed or not when an expression list is used.
(17.5) — Otherwise, if the destination type is an array, the object is initialized as follows. Let x1, . . . , xk be the elements of the expression-list. If the destination type is an array of unknown bound, it is defined as having k elements. Let n denote the array size after this potential adjustment. If k is greater than n, the program is ill-formed. Otherwise, the ith array element is copy-initialized with xi for each 1 ≤ i ≤ k, and value-initialized for each k < i ≤ n. For each 1 ≤ i < j ≤ n, every value computation and side effect associated with the initialization of the ith element of the array is sequenced before those associated with the initialization of the jth element.
Rereading the provided quote especially its this part
the ith array element is copy-initialized with xi for each 1 ≤ i ≤ k
It seems that such an initialization like this
int a[][3]( 1, 2, 3, { 4, 5, 6 } );
is indeed incorrect because the element of the array a is in turn an array of the type int[3] and to initialize it in this case you need to use a braced-init-list. After that within the braced-init-list the brace elision will be accepted. That is you may not initialize an array with an expression.
Here is an example that confirms that and is compiled by the C++ compiler of MS VS 2019
int e[][2][3]( { 1, 2, 3, { 4, 5, 6 } }, { 6, 5, 4, 3, 2, 1 } );
Within the braced-init-lists the brace elision may be used.