Why does Clang 12 refuse to initialize aggregates in the C++20 way?

Question

As far as I understand, the following program should work in C++20 mode:

#include <vector> struct B{ int a0, a1; }; int main() { std::vector<B> bs; bs.emplace_back( 0, 0 ); }

And it really does in Visual Studio 2019 and gcc 11. But not in clang 12, which produces the error:

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/alloc_traits.h:514:4: error: no matching function for call to 'construct_at' std::construct_at(__p, std::forward<_Args>(__args)...); ^~~~~~~~~~~~~~~~~

In online compiler: https://gcc.godbolt.org/z/GzccTWc5z

Is this because clang does not fully support C++20 yet?

Cody Gray · Accepted Answer · 2021-06-16 16:45:39Z

This is a C++20 feature that allows aggregate initialization through the standard constructor syntax, rather than the typical braced-list initialization syntax. (Note that this only works if the parameters cannot be used in a valid call to a default or copy/move constructor. If they could, that would be called instead of performing aggregate initialization.)

According to the official C++ Support in Clang page, Clang does not yet support parenthesized initialization of aggregates (which is P0960R3 and P1975R0). This is as of Clang version 13.

A support matrix for C++20 features is also maintained on cppreference.com. This shows that support for P0960R3 is as follows:

GCC: supported as of version 10
MSVC: supported as of version 19.28
EDG eccp: supported as of version 5.1
Clang and Apple Clang: unsupported
Intel ICC: unsupported

"Note that this only works if the parameters cannot be used in a valid call to a default or copy/move constructor. If they could, that would be called instead of performing aggregate initialization.". At least that is consistent with brace-init which behaves in the same way AFAIK. Or is there a difference here? One thing comes to mind: I suspect if the initializer has an implicit conversion to the aggregate type, things could behave different.
Note that Clang 16 now supports it, but it's still unavailable on Apple clang.

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