Testcase:

    extern long double l;

    struct S {
        S(int) {}
    };

    S s[] = { l };


This is supposed to be ill-formed because of narrowing. GCC does not reject the code, nor it warns (as it usually does for narrowing).

The relevant rule is in [dcl.init.aggr]:

https://eel.is/c++draft/dcl.init.aggr#4.2.sentence-2

> 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.


That rule has been changed in C++20 with designated initializers, but the same one was also there for C++17 and before, e.g. https://timsong-cpp.github.io/cppwp/n4659/dcl.init.aggr#3


For comparison, Clang accepts with no warnings, but MSVC warns about narrowing.