Clang C++20 Feature: P0960R3 -Allow initializing aggregates from a parenthesized list of values

[tstellar]

Blocks #46701

---

id: P0960R3
paper: https://wg21.link/p0960r3

[llvmbot]

@llvm/issue-subscribers-c-20

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[0x59616e]

Candidate patch : https://reviews.llvm.org/D129531

[danakj]

Thank you for working on this. One thing that was not fixed is templated classes.

https://godbolt.org/z/6a6Ws674o

The following works in GCC and MSVC but not on Clang trunk yet:

struct S {
    int i;
    float j;
};

template <class T>
struct ST {
    T t;
};

int main() {
    auto s = S(1, 2.f);  // works
    auto st = ST(1);  // no work
}

[alanzhao1]

That specific example is out of scope of P0960R3 because it's covered by two other C++20 feature requests - P1816R0 and P2082R1, both of which track class template argument deduction for aggregates. Support for this in Clang is tracked in #54049 and #54050

[danakj]

Thanks, one more thing I ran into: A destructor breaks the aggregate init for Clang, though not for MSVC and GCC.

https://godbolt.org/z/sbj8hTKWb

struct Raii final {
    ~Raii() {}
    int i;
};

int main() {
    auto r = Raii(2);
}

<source>:7:14: error: no matching conversion for functional-style cast from 'int' to 'Raii'
    auto r = Raii(2);
             ^~~~~~
<source>:1:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const Raii' for 1st argument
struct Raii final {
       ^
<source>:1:8: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
1 error generated.

[danakj]

Another issue is caused somehow by a concept-guarded conversion operator on the type of the aggregate's member.

https://godbolt.org/z/sMY7WfGhx

#include <concepts>

struct i32 {
    int i = 0;
    
    template <std::same_as<int> T>
    operator T() const { return i; }  // breaks init of S()
};

int main() {
    struct S {
        i32 i;
    };

    auto s1 = S(i32(2));
}

<source>:15:15: error: no matching conversion for functional-style cast from 'i32' to 'S'
    auto s1 = S(i32(2));
              ^~~~~~~~
<source>:11:12: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'i32' to 'const S' for 1st argument
    struct S {
           ^
<source>:11:12: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'i32' to 'S' for 1st argument
    struct S {
           ^
<source>:11:12: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
<source>:7:5: note: candidate template ignored: constraints not satisfied [with T = S]
    operator T() const { return i; }  // breaks init of S()
    ^
<source>:6:15: note: because 'std::same_as<S, int>' evaluated to false
    template <std::same_as<int> T>
              ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.0/../../../../include/c++/13.0.0/concepts:63:9: note: because '__detail::__same_as<S, int>' evaluated to false
      = __detail::__same_as<_Tp, _Up> && __detail::__same_as<_Up, _Tp>;
        ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.0/../../../../include/c++/13.0.0/concepts:57:27: note: because 'std::is_same_v<S, int>' evaluated to false
      concept __same_as = std::is_same_v<_Tp, _Up>;
                          ^
1 error generated.

[alanzhao1]

The previous two issues should be resolved by https://reviews.llvm.org/D140327

[danakj]

Awesome, thank you

[alanzhao1]

Reopening as this was reverted by 4e02ff2 due to #59675