Why does including break structured bindings in GCC?

Question

Consider:

struct Point { int x, y; };

int main()
{
    const auto [x, y] = Point{};
}

This code compiles fine with gcc 7.1 in C++17 mode, howe

Answer 1

This is compiler bug 78939. Although it's a bit more complicated than that - there were a few issues between the core language and the library that were mutually contradictory (GB 20, LWG 2770, and LWG 2446), which lead to the kind of behavior that gcc/libstdc++ exhibit here. It is certainly intended that the code work with or without #include , it's just a matter of the standard wording having gotten there properly.

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Yes, classes with all public non-anonymous union members should be usable in structured bindings declarations per [dcl.struct.bind]/4:

Otherwise, all of E's non-static data members shall be public direct members of E or of the same unambiguous public base class of E, E shall not have an anonymous union member, and the number of elements in the identifier-list shall be equal to the number of non-static data members of E. Designating the non-static data members of E as m0, m1, m2, ... (in declaration order), each vi is the name of an lvalue that refers to the member mi of e and whose type is cv Ti, where Ti is the declared type of that member; the referenced type is cv Ti. The lvalue is a bit-field if that member is a bit-field. [ Example:

struct S { int x1 : 2; volatile double y1; };
S f();
const auto [ x, y ] = f();

This is completely unrelated to the inclusion of , nothing in this code depends on any library functionality - the members are grabbed directly, and not via the get/tuple_size mechanism.