SFINAE and E1.E2

The other day on Slack someone asked whether there was anything that was easier to express using classic SFINAE techniques than using C++20 Concepts techniques. I suggested

template<class T, class U>
auto plus(T t, U u) -> decltype(t + u)
    return t + u;

This expresses the SFINAE constraint that this function shall not participate in overload resolution unless t + u is a well-formed expression. To do the same with Concepts, idiomatically, you might say something like

template<class T, class U>
concept Plussable = requires(T t, U u) { t + u; };

template<class U, Plussable<U> T>
decltype(auto) plus(T t, U u)
    return t + u;

And even then, I wondered, did the Concepts version really capture everything about the original? The original expresses as part of its signature that its return type is the type of t+u; could this have a SFINAE effect?

I think the answer is “no,” in the case of t+u. But consider:

template<class T>
auto dot1(T t) -> decltype(t.u)
    return t.u;

template<class T> requires requires(T t) { t.u; }
decltype(auto) dot2(T t)
    return t.u;

void dot1(...);
void dot2(...);

struct A {
    static int u[];
struct A10 {
    static int u[10];
struct F {
    static int u(int);
struct MF {
    int u(int);

Godbolt yields the following results:

dot1(A()) dot1(...) dot1(...) dot1<A> -> int[] dot1(...)
dot2(A()) error error error N/A
dot1(A10()) dot1(...) dot1(...) dot1<A10> -> int[10] dot1(...)
dot2(A10()) error error error N/A
dot1(F()) dot1(...) dot1<F>() -> int(&)(int) error dot1(...)
dot2(F()) error dot2<F>() -> int(&)(int) error N/A
dot1(MF()) dot1(...) dot1(...) error dot1(...)
dot2(MF()) dot2(...) error error N/A

I agree with GCC’s behavior in all of these cases.

  • dot1<T> should SFINAE away whenever auto (T) -> decltype(t.u) is not a well-formed function type, and that includes whenever decltype(t.u) is an array, function, or member-function type.

  • Contrariwise, dot2<T> participates in overload resolution whenever t.u is well-formed, and so it’s correct for each of its uses to give a hard (non-SFINAE-friendly) error.

The final case, dot2(MF()), is different from the rest. Here, the expression t.u itself is ill-formed per [expr.ref]/6.3.2:

[When E2 refers to a non-static member function], E1.E2 is a prvalue. The expression can be used only as the left-hand operand of a member function call.

All four compilers agree that in normal code, even discarding the value counts as a “use”:

struct MF { int u(int); };
void f1() { MF mf; mf.u; }  // error
void f2() { MF mf; (void)mf.u; }  // error

However, every compiler has its own opinion about how this plays out in a requires-expression. Godbolt:

template<class T> concept C1 =
    requires (T t) { t.u; };
template<class T> concept C2 =
    requires (T t) { (void)t.u; };

static_assert(C1<MF> && C2<MF>);   // MSVC
static_assert(C1<MF> && !C2<MF>);  // Clang
static_assert(!C1<MF> && !C2<MF>); // GCC

Again, my impression is that GCC is correct and the others are (at time of writing) wrong.

Posted 2020-11-01