Failed Template Deduction in C++20 Designated Initializers

[marstaik]

Tested with clang 14.0

When a template field is initialized via designated initializers, clang fails to deduce the final specialization.

Code example:

template<typename T>
struct Foo {
    int foo;
    T bar;
};

int main() {
    constexpr auto foo = Foo {
        .foo = 1,
        .bar = +[]() { return 5; }
    };

    constexpr auto eval = foo.bar();
    
    return 0;
}

Note that this works succesfully in MSVC and GCC:

MSVC: https://godbolt.org/z/qr5a56xbs
GCC: https://godbolt.org/z/sqc4r1Eb4
clang: https://godbolt.org/z/P1c3v7Gfj

The compiler reports:

x86-64 clang 14.0.0 - 358ms

<source>:8:26: error: no viable constructor or deduction guide for deduction of template arguments of 'Foo'
    constexpr auto foo = Foo {
                         ^
<source>:2:8: note: candidate function template not viable: requires 1 argument, but 2 were provided
struct Foo {
       ^
<source>:2:8: note: candidate function template not viable: requires 0 arguments, but 2 were provided
1 error generated.
ASM generation compiler returned: 1
<source>:8:26: error: no viable constructor or deduction guide for deduction of template arguments of 'Foo'
    constexpr auto foo = Foo {
                         ^
<source>:2:8: note: candidate function template not viable: requires 1 argument, but 2 were provided
struct Foo {
       ^
<source>:2:8: note: candidate function template not viable: requires 0 arguments, but 2 were provided
1 error generated.
Execution build compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-format

[llvmbot]

@llvm/issue-subscribers-c-20

[marstaik]

@EugeneZelenko I think this is more of a clang-frontend issue than a clang-format issue :P

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[EugeneZelenko]

@marstaik: Sorry for my mistake. Fixed.

[schwarzschild-radius]

I think it is because of missing deduction candidate. Here is a smaller repro (https://godbolt.org/z/b4v14T74d)

template<typename T>
struct Foo {
    int foo;
    T bar;
};

int main() {
    auto foo = Foo {1, 0.5};
    return 0;
}

[alanzhao1]

Fixing this is going to be somewhat complicated

Currently, template argument deduction is centered around functions. Functions cover things such as constructors. Aggregate initializers, however, do not fit this model, so I think there might be a good amount of refactoring necessary to resolve this.

[danakj]

Even smaller repro: https://godbolt.org/z/xqbde4hfM

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

S s{.t = int(3)};

[alanzhao1]

I believe this bug is a duplicate of #54050 and #54049, both of which are track the implementation of class template argument deduction for aggregates as described in P1816R0 and P2082R1.

[yuanfang-chen]

duplicates #54049 and #54050