`<array>`: CTAD and element count limit

[tiagomacarios]

The pseudo code below when compiled with clang errors out.

static constexpr const std::array arr = { 49000 items };

error:

<compiler>\lib\native\include\type_traits:40:27: error: recursive template instantiation exceeded maximum depth of 1024 [clang-diagnostic-error]
   40 |     using type = typename _Conjunction<static_cast<bool>(_Next::value), _Next, _Rest...>::type;
      |                           ^
<compiler>\lib\native\include\type_traits:40:27: note: in instantiation of template class 'std::_Conjunction<true, std::is_same<unsigned int, unsigned int>, ..., std::is_same<unsigned int, unsigned int>>' requested here
  584 |         conjunction_v<is_same<_First, _Rest>...>, "N4950 [array.cons]/2: Mandates: (is_same_v<T, U> && ...) is true.");
      |         ^
<compiler>\lib\native\include\array:589:43: note: in instantiation of template class 'std::_Enforce_same<unsigned int, ..., unsigned int>' requested here
  589 | array(_First, _Rest...) -> array<typename _Enforce_same<_First, _Rest...>::type, 1 + sizeof...(_Rest)>;
      |                                           ^
<file>:11:35: note: while substituting deduced template arguments into function template '<deduction guide for array>' [with _First = unsigned int, _Rest = <unsigned int, ..., unsigned int>]
   11 | static constexpr const std::array arr = {
      |  

[frederick-vs-ja]

Does this compile if you replace conjunction_v<is_same<_First, _Rest>...> with (is_same_v<_First, _Rest> && ...)?

[StephanTLavavej]

Even with a fold expression, Clang will eventually say:

D:\GitHub\STL\stl\inc\array(582,38): fatal error: instantiating fold expression with 99999 arguments exceeded expression nesting limit of 256
  582 |         (is_same_v<_First, _Rest> && ...), "N4950 [array.cons]/2: Mandates: (is_same_v<T, U> && ...) is true.");
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
[...]
D:\GitHub\STL\stl\inc\array(582,38): note: use -fbracket-depth=N to increase maximum nesting level

This is apparently because Clang emits nested parentheses as it expands a fold expression, eventually reaching a limit.

There doesn't seem to be an easy way for the STL to avoid compiler limits here. I'd recommend asking the Clang devs to implement fold expressions more robustly. In the meantime, you should probably explicitly specify your element count.

My repro:

#include <array>
#include <type_traits>
using namespace std;

#define Ten1 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
#define Ten2 Ten1, Ten1, Ten1, Ten1, Ten1, Ten1, Ten1, Ten1, Ten1, Ten1
#define Ten3 Ten2, Ten2, Ten2, Ten2, Ten2, Ten2, Ten2, Ten2, Ten2, Ten2
#define Ten4 Ten3, Ten3, Ten3, Ten3, Ten3, Ten3, Ten3, Ten3, Ten3, Ten3
#define Ten5 Ten4, Ten4, Ten4, Ten4, Ten4, Ten4, Ten4, Ten4, Ten4, Ten4

constexpr array arr{Ten5};

static_assert(is_same_v<decltype(arr), const array<int, 100'000>>);

int main() {}

[frederick-vs-ja]

I'd recommend asking the Clang devs to implement fold expressions more robustly.

This seems already done in LLVM-132021 (landed in Clang 21). However, it seems that the new limit is not yet sufficient.

It seems that Clang (trunk) still crashes on CTAD even if we implement the deduction guide without Mandates.

[frederick-vs-ja]

It seems that Clang (trunk) still crashes on CTAD even if we implement the deduction guide without Mandates.

This bug is tracked in LLVM-119600. MSVC seems to crash after the same limit (65535 template parameters), while EDG can handle at least 100000 template parameters.
I'm trying to fix original issue where there's "merely" ~49000 template parameters in CTAD.

[DanielJump]

I can workaround this with std::to_array. I noticed this because clang-tidy was failing at 1000 elements 🙂. Get Outlook for Android<https://aka.ms/AAb9ysg>

…

________________________________ From: A. Jiang ***@***.***> Sent: Sunday, August 3, 2025 9:15:50 AM To: microsoft/STL ***@***.***> Cc: Manual ***@***.***> Subject: Re: [microsoft/STL] `<array>`: CTAD and element count limit (Issue #5665) It seems that Clang (trunk) still crashes on CTAD even if we implement the deduction guide without Mandates. This bug is tracked in LLVM-119600<https://llvm.org/PR119600>. MSVC seems to crash after the same limit (65535 template parameters), while EDG can handle at least 100000 template parameters. I'm trying to fix original issue where there's "merely" ~49000 template parameters in CTAD. — Reply to this email directly, view it on GitHub<#5665 (comment)> or unsubscribe<https://github.com/notifications/unsubscribe-auth/ACNNQ3BBHTN3H7IZZVTOG733LYYTNBFKMF2HI4TJMJ2XIZLTSOBKK5TBNR2WLJDUOJ2WLJDOMFWWLO3UNBZGKYLEL5YGC4TUNFRWS4DBNZ2F6YLDORUXM2LUPGBKK5TBNR2WLJDUOJ2WLJDOMFWWLLTXMF2GG2C7MFRXI2LWNF2HTAVFOZQWY5LFUVUXG43VMWSG4YLNMWVXI2DSMVQWIX3UPFYGLLDTOVRGUZLDORPXI6LQMWWES43TOVSUG33NNVSW45FGORXXA2LDOOJIFJDUPFYGLKTSMVYG643JORXXE6NFOZQWY5LFVEZDANBVHEZTQMRVQKSHI6LQMWSWS43TOVS2K5TBNR2WLKRTGI4DMMJTGU3TQNVHORZGSZ3HMVZKMY3SMVQXIZI>. You are receiving this email because you are subscribed to this thread. Triage notifications on the go with GitHub Mobile for iOS<https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android<https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.