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[BUG] UFCS with variable template in scope fails #807
Description
Title: UFCS with variable template in scope fails.
Description:
std::vector<int>().empty() will fail given using std::views::empty;.
That should end up calling the member empty of std::vector.
But the UFCS macro fails on the free function branch.
All compilers fail: https://compiler-explorer.com/z/bWejqfodq.
With #506, MSVC starts accepting: https://compiler-explorer.com/z/TYaMW7av6.
Minimal reproducer (https://cpp2.godbolt.org/z/7nfzr7zYx):
#include <ranges>
#include <vector>
using std::views::empty;
auto _ = std::vector<int>().empty();
main: () = {
_ = std::vector<int>().empty();
}
Commands:
cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cppExpected result: A well-formed program that calls the member empty of std::vector.
Actual result and error:
Cpp2 lowered to Cpp1:
//=== Cpp2 type declarations ====================================================
#include "cpp2util.h"
//=== Cpp2 type definitions and function declarations ===========================
#include <ranges>
#include <vector>
using std::views::empty;
auto _ = std::vector<int>().empty();
auto main() -> int;
//=== Cpp2 function definitions =================================================
auto main() -> int{
static_cast<void>(CPP2_UFCS_0(empty, std::vector<int>()));
}Output:
main.cpp2:6:33: error: use of variable template 'empty' requires template arguments
6 | static_cast<void>(CPP2_UFCS_0(empty, std::vector<int>()));
| ^
/opt/compiler-explorer/clang-trunk-20231104/bin/../include/c++/v1/__ranges/empty_view.h:45:36: note: template is declared here
44 | template <class _Tp>
| ~~~~~~~~~~~~~~~~~~~~
45 | inline constexpr empty_view<_Tp> empty{};
| ^
1 error generated.
Activity
JohelEGP commentedon Nov 5, 2023
I noticed the same failure if you attempt to call
std::vector<int>().std::vector<int>::empty()(https://cpp2.godbolt.org/z/c5veMnjMe).Branch
main: https://compiler-explorer.com/z/9oa1Ga3Tx (but see #555).#506: https://compiler-explorer.com/z/qvK7E4Kj8.
Except that Clang's error message requests for disambiguating
template.This issue came up while implementing
@rule_of_zeroat #797.Fortunately, I could fix it with a local
using std::ranges::empty; // Pending #506.(
std::views::emptyactually came fromusing namespace std::views;).So it doesn't directly use
std::vector::empty.I was fortunate enough to have such a fix-hack at hand.
JohelEGP commentedon Nov 7, 2023
I think this one really is IFNDR.
Reading https://eel.is/c++draft/algorithms.requirements#2
reminded me that the else branch,
although never taken for
std::vector,will always be ill-formed for all invocations (because
std::views::emptyneeds a template argument).JohelEGP commentedon Nov 7, 2023
IIRC from my analysis at #307, it's that this can't be solved without Cpp1 reflection.
JohelEGP commentedon Nov 9, 2023
The same seems to happen for
operator(): https://compiler-explorer.com/z/65Y5rh7P5.JohelEGP commentedon Dec 13, 2023
The one case we can handle is when source local name lookup results in a variable.
If it isn't templated,
func<targs>(obj)can't be valid, so the only option is the member call.See https://cpp2.godbolt.org/z/T6sdezYKh: