Clang参数包与成员函数指针类型推导问题求助

Hey there! Let's dig into why you're hitting this issue with Clang and parameter packs—you're right that parameter packs shouldn't require default values, so this error is definitely pointing to a subtle misstep in how your code uses them.

First off, the error snippet you shared (In file included from input_line_9:1:) is just the header of Clang's error output; the actual critical details are almost certainly in the lines that follow. But based on your description, this is almost always tied to incorrectly using or failing to expand a parameter pack in your code.

Common Scenario That Triggers This Error #

Let's look at a typical mistake that would cause Clang to behave like this:

// Wrong: Treating the parameter pack as a single template parameter
template <typename... Args>
void my_function(Args args) {
    // This doesn't expand the pack—Clang sees `Args` as a single unknown type
}

// Correct: Properly expanding the parameter pack
template <typename... Args>
void my_function(Args... args) {
    // Now the pack is expanded into multiple function parameters
}

In the wrong example, Clang doesn't recognize Args as a parameter pack here (since you omitted the ... when using it in the function parameter list). Instead, it treats it as a regular template parameter that needs to be specified—but since it's part of a pack, there's no way to resolve it, leading to an error that may reference missing default values (even though that's not the actual fix).

Another Possible Case: Class Template Member Issues #

If your code involves a class template with a parameter pack, a similar mistake could happen with constructors or member functions:

// Wrong
template <typename... Args>
struct MyClass {
    // Again, `Args` here is treated as a single type, not a pack
    MyClass(Args);
};

// Correct
template <typename... Args>
struct MyClass {
    MyClass(Args...);
};

Why This Happens #

Parameter packs themselves don't need default values, but when you don't properly use the ... syntax to expand them, Clang interprets the pack identifier as a regular, unbound template parameter. Since that parameter can't be deduced or explicitly set in your code, Clang throws an error that may mention missing default values as a possible workaround—even though the real fix is correcting how you expand the pack.

Next Steps to Fix It #

• Double-check every place you use your parameter pack: make sure you add ... whenever you need to expand it (whether in template parameters, function parameters, or when using the pack in expressions).
• Pull up the full error message from Clang—it will point directly to the line where the pack is being misused, which makes debugging way easier.

内容的提问来源于stack exchange，提问作者jetstream