c++ - 类型推导 lambda 引用资料

Question

我看到了this answer但我不满意:“auto”的类型推导不考虑默认参数的标准在哪里写的？

int main() {
    auto lambda = [] (auto subl = [] {}) {};
    lambda();
}

Answer 1

auto 根据模板参数推导使用规则([dcl.spec.auto]/p7):

If the placeholder is the auto type-specifier, the deduced type is determined using the rules for template argument deduction.

当默认参数的类型是推导模板参数时，默认参数是明确的非推导上下文 ([temp.deduct.type]/p5):

The non-deduced contexts are:

— The nested-name-specifier of a type that was specified using a qualified-id.

— The expression of a decltype-specifier.

— A non-type template argument or an array bound in which a subexpression references a template parameter.

— A template parameter used in the parameter type of a function parameter that has a default argument that is being used in the call for which argument deduction is being done.