Class 模板参数推导 - clang 和 gcc 不同

Question

以下代码compiles with gcc but not with clang：

template<typename T>
class number {
    T num;
public:
    number(T num = 0): num(num) {}
    
    template<typename T1, typename T2>
    friend auto add(T1 a, T2 b);
};

template<typename T1, typename T2>
auto add(T1 a, T2 b) {
    // this compiles with both:
        // return number<T1>{a}.num + number<T2>{b}.num;
    // this compiles only with gcc:
    return number{a}.num + number{b}.num; // <== clang is unhappy here
}

int main() {
    auto result = add(1.0, 2.0);
}

clang 提供的编译错误（版本 10.0.0 with -std=c++20）：

error: member reference base type 'number' is not a structure or union

    return number{a}.num + number{b}.num;

           ~~~~~~~~~^~~~

error: member reference base type 'number' is not a structure or union

    return number{a}.num + number{b}.num;

                           ~~~~~~~~~^~~~

正确的行为是什么？

Answer 1

正如 Barry 在评论中提到的，这是一个 clang 错误 44468 (and 47870)。

错误已修复。代码现在可以编译并运行良好 in Clang trunk.

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