<array>: std::array<T,0> doesn't compile - when type is not default constructible

[AlexGuteniev]

Describe the bug
std::array<T,0> doesn't compile - when type has private default constructor, deleted default constructor, or no default constructor.

If there's a default constructor, it is called once during std::array<T,0> construction.

According to LWG-2157, there should be a single element of unspecificed type, and such array should be still default constructible.

Command-line test case

D:\Temp2>type repro.cpp
#include <array>

struct NonDefault {
    NonDefault() = delete;
    NonDefault(int) {}
};

std::array<NonDefault, 0> a;

int main() {
}
D:\Temp2>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
.\repro.cpp(8): error C2280: 'std::array<NonDefault,0>::array(void)': attempting to reference a deleted function
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\array(770): note: compiler has generated 'std::array<NonDefault,0>::array' here
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\array(770): note: 'std::array<NonDefault,0>::array(void)': function was implicitly deleted because a data member invokes a deleted or inaccessible function 'NonDefault::NonDefault(void)'
.\repro.cpp(4): note: 'NonDefault::NonDefault(void)': function was explicitly deleted

Expected behavior
Test compiles, links and runs.

STL version
Microsoft Visual Studio Professional 2019 Preview
Version 16.7.0 Preview 3.1

Additional context

• Skipped libcxx test:
  

  STL/tests/libcxx/expected_results.txt

  Lines 576 to 581 in 06827fe

  	# STL bug: VSO-207715 We reject array<NoDefault, 0>.
  	std/containers/sequences/array/array.cons/implicit_copy.pass.cpp FAIL
  	std/containers/sequences/array/array.cons/initialization.pass.cpp FAIL
  	std/containers/sequences/array/array.data/data_const.pass.cpp FAIL
  	std/containers/sequences/array/array.data/data.pass.cpp FAIL
  	std/containers/sequences/array/iterators.pass.cpp FAIL

This item is also tracked on Developer Community as DevCom-329888, and by Microsoft-internal VSO-207715 / AB#207715.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

[StephanTLavavej]

Thanks, I've linked the Microsoft-internal bug to this issue!

Note that LWG-2157 is still Open - it has not been resolved in the Working Paper. However, we've received enough reports of this that I think the direction is clear.

[CaseyCarter]

IIRC the remaining issue with the PR of LWG-2157 was that I wanted the note ("[Note: Therefore begin() does not return a value-initialized iterator.—end note]" struck on the basis that it's UB for users to compare the result of calling begin on distinct ranges unless we explicitly specify otherwise. I want to implement array<T, 0> with size 0 instead of sizeof(T) which it must otherwise be if we need to return a unique T* value from begin. That said, notes are non-normative so I'm inclined to implement what I like when we break ABI regardless of how LWG-2157 is resolved.

[AlexGuteniev]

There are some test cases already:

STL/tests/libcxx/expected_results.txt

Lines 574 to 583 in 5be7d49

	# STL bug: We allow fill() and swap() for array<const T, 0>.
	std/containers/sequences/array/array.fill/fill.fail.cpp FAIL
	std/containers/sequences/array/array.swap/swap.fail.cpp FAIL
	# STL bug: VSO-207715 We reject array<NoDefault, 0>.
	std/containers/sequences/array/array.cons/implicit_copy.pass.cpp FAIL
	std/containers/sequences/array/array.cons/initialization.pass.cpp FAIL
	std/containers/sequences/array/array.data/data_const.pass.cpp FAIL
	std/containers/sequences/array/array.data/data.pass.cpp FAIL
	std/containers/sequences/array/iterators.pass.cpp FAIL