[dcl.init.aggr] Narrowing Conversions in Brace-Enclosed initializer-lists for Aggregate Initialization

[ckwastra]

Full name of submitter: Vincent X

Reference: [dcl.init.aggr]

Issue description: The corrent wording of [dcl.init.aggr]/4.3:

• Otherwise, the initializer list is a brace-enclosed initializer-list. If an initializer-clause appertains to the aggregate element, then the aggregate element is copy-initialized from the initializer-clause. Otherwise, the aggregate element is copy-initialized from a brace-enclosed initializer-list consisting of all of the initializer-clauses that appertain to subobjects of the aggregate element, in the order of appearance.

lacks a prohibition on narrowing conversions. This branch was split from [dcl.init.aggr]/4.2 (emphasis mine):

• Otherwise, the element is copy-initialized from the corresponding initializer-clause or is initialized with the brace-or-equal-initializer of the corresponding designated-initializer-clause. If that initializer is of the form assignment-expression or = assignment-expression and a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed.

by P3106R1, but that revision didn't retain the wording prohibiting narrowing conversions, as shown above.

Suggested resolution: Amend [dcl.init.aggr]/4.3 as follows:

• Otherwise, the initializer list is a brace-enclosed initializer-list. If an initializer-clause appertains to the aggregate element, then the aggregate element is copy-initialized from the initializer-clause. If that initializer is of the form assignment-expression and a narrowing conversion ([dcl.init.list]) is required to convert the expression, the program is ill-formed. Otherwise, the aggregate element is copy-initialized from a brace-enclosed initializer-list consisting of all of the initializer-clauses that appertain to subobjects of the aggregate element, in the order of appearance.

[t3nsor]

I wonder why we don't just consolidate all the prohibitions on narrowing into a single sentence? Maybe something like: "If a narrowing conversion is applied to an element of a braced-init-list or to the initializer-clause of a designated-initializer-clause, the program is ill-formed"?

[dangelog]

The current wording makes something like this well-formed

extern long double l;
struct S { S(int); };
S s[] = { l };

This used to be ill-formed before P3106, because of the narrowing conversion.

I don't think making this well-formed was intentional. This breaks detectors of narrowing conversions; for instance, it breaks std::variant's construction and assignment, which rely on a construct similar to the above code (for example: https://eel.is/c++draft/variant.ctor#14.sentence-1 ), cf. P0870 and P3146.