C 20 lambda in template: unable to deduce ‘auto*’ from lambda-CodePudding

Given the following simple wrapper struct (inspired by this answer):

template <auto* F> struct Wrapper;

template <class Ret, class... Args, auto (*F)(Args...) -> Ret>
struct Wrapper<F>
{
    auto operator()(Args... args) const
    {
        return F(args...);
    }
};

The following works:

int this_works(){
    return 42;
}
int main(){
    return Wrapper<this_works>()();
}

But I want this, using c 20:

int main(){
    return Wrapper<[](){return 42;}>()();
}

g -11 --std=c 20 and clang 13 --std=c 20 both complain about the latter with some hard to decipher error messages, including:

mismatched types ‘auto*’ and ‘main()::<lambda()>
error: non-type template parameter 'F' with type 'auto *' has incompatible initializer

Is there a way to make the second example work? I tried a constexpr function pointer to the lambda but it complained about it having no linkage ...

CodePudding user response：

Wrapper expects function pointer, but template argument deduction won't consider implicit conversion (from lambda without capture to function pointer).

You can convert the lambda to function pointer explicitly:

int main(){
    return Wrapper<static_cast<int(*)()>([](){return 42;})>()();
}

int main(){
    return Wrapper< [](){return 42;}>()();
}

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