CWG3030 [expr.type.conv] lacks the wording to specify the number of elements of an unknown bound array type #509

Full name of submitter (unless configured in github; will be published with the issue): Jim X

Consider this case:

int main(){
   using type = int[];
   type{1,2,3};
}

[dcl.array] p7 says:

An array bound may also be omitted when an object (but not a non-static data member) of array type is initialized and the declarator is followed by an initializer ([dcl.init], [class.mem], [expr.type.conv], [expr.new]).

The wording does not apply to this example because there is no declaration of an object, the expression is a prvalue. [dcl.init], [expr.new] specify how the array bound is calculated

[dcl.init.aggr] p10

An array of unknown bound initialized with a brace-enclosed initializer-list containing n initializer-clauses is defined as having n elements ([dcl.array]).

[expr.new] p7

If the type-id or new-type-id denotes an array type of unknown bound ([dcl.array]), the new-initializer shall not be omitted; the allocated object is an array with n elements, where n is determined from the number of initial elements supplied in the new-initializer ([dcl.init.aggr], [dcl.init.string]).

However, the nominated clause [expr.type.conv] in [dcl.array] does not describe how the array bound is calculated.

Suggested Resolution

Appending a similar wording in [expr.type.conv] p2

If the specified type is an array of an unknown bound, the initializer shall not be an empty braced-init-list {}, and the bound is determined by the number of elements of the initializer.