https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77825
Bug ID: 77825
Summary: return type deduction regression in 7.0
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Can't come up with a more reduced example, but consider this code that makes a
Y combinator version of factorial and calls it:
#include <type_traits>
#include <utility>
template <class F>
struct y_combinator {
F f;
template <class... Args>
decltype(auto) operator()(Args&&... args) const
{
return f(*this, std::forward<Args>(args)...);
}
};
template <class F>
y_combinator<std::decay_t<F>> make_y_combinator(F&& f) {
return {std::forward<F>(f)};
}
int main() {
auto factorial = make_y_combinator([](auto fact, int n) -> int {
if (n == 0) return 1;
else return n * fact(n - 1);
});
factorial(5);
}
The code is well formed, operator() returns decltype(auto), but f is a lambda
that explicitly returns int. The code compiles in 4.9.3, 5.3, and 6.1, but
fails on trunk 7.0 with:
prog.cc: In instantiation of 'main()::<lambda(auto:1, int)> [with auto:1 =
y_combinator<main()::<lambda(auto:1, int)> >]':
prog.cc:11:52: required from 'decltype(auto)
y_combinator<F>::operator()(Args&& ...) const [with Args = {int}; F =
main()::<lambda(auto:1, int)>]'
prog.cc:24:16: required from here
prog.cc:22:36: error: use of 'decltype(auto) y_combinator<F>::operator()(Args&&
...) const [with Args = {int}; F = main()::<lambda(auto:1, int)>]' before
deduction of 'auto'
else return n * fact(n - 1);
~~~~^~~~~~~
