[llvm-bugs] [Bug 46137] New: std::array<T, 0> is invalid if T has a non-trivial destructor

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46137

            Bug ID: 46137
           Summary: std::array<T, 0> is invalid if T has a non-trivial
                    destructor
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: leonardchan at google.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Currently, it seems we cannot create an empty std::array of type S if S has a
non-trivial destructor:

```
#include <array>

struct S {
  ~S(){}
};

template <typename T>
S DoSomething(const T&) {
  return S();
}

template <typename... As>
void other_func(As&&... args) {
  (void)(std::array<S, sizeof...(As)>{DoSomething(args)...});
}

void func() {
  other_func();  // error
  other_func(0); // ok
  std::array<S, 0> x;  // error
  std::array<S, 1> y;  // ok
  (void)x;
  (void)y;
}
```

This compiled with ToT clang (`clang++ -std=c++17 -c test.cc`) returns

```
test.cc:20:20: error: call to implicitly-deleted default constructor of
'std::array<S, 0>'
  std::array<S, 0> x;  // bad
                   ^
/usr/local/google/home/leonardchan/llvm-monorepo/llvm-build-1-master-fuchsia-toolchain/install/bin/../include/c++/v1/array:252:7:
note: default constructor of 'array<S, 0>' is implicitly deleted because field
'__w' has a deleted destructor
    } __w;
      ^
/usr/local/google/home/leonardchan/llvm-monorepo/llvm-build-1-master-fuchsia-toolchain/install/bin/../include/c++/v1/array:248:9:
note: explicitly defaulted function was implicitly deleted here
        ~__wrapper() = default;
        ^
/usr/local/google/home/leonardchan/llvm-monorepo/llvm-build-1-master-fuchsia-toolchain/install/bin/../include/c++/v1/array:251:13:
note: destructor of '__wrapper' is implicitly deleted because variant field
'__t' has a non-trivial destructor
        _Tp __t;
            ^
test.cc:20:20: error: attempt to use a deleted function
  std::array<S, 0> x;  // bad
                   ^
... and a couple more
```

`std::array<S, 0>` should be valid regardless of constraints specified about T.

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