如果使用非模板类型，则为Constexpr [英] Constexpr if with non-template types

问题描述

#include <iostream>

int foo(int x) {
    if constexpr (std::is_same_v<decltype(x), std::string>) {
        x = std::string();
    }
}

int main(void)
{ return 0; }

此代码无法在GCC 7或Clang 5上编译:

This code doesn't compile on either GCC 7 nor Clang 5:

error: cannot convert ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘int’ in assignment
         x = std::string();

由于引用的行在constexpr中(如果分支的求值应为false)，程序是否应该编译正常?

Since the referenced line is in a constexpr if branch that should evaluate to false, shouldn't the program compile fine?

推荐答案

if constexpr规范定义了废弃的语句.然后，它继续定义当结果不依赖于实例化后的 时，不实例化被丢弃的语句.暗示是语句在模板实例化期间被丢弃.此外，仅在条件值取决于模板参数的情况下才丢弃该语句.

The if constexpr specification defines the discarded statement. It then goes on to define that the discarded statement is not instantiated when the result is not value-dependent after instantiation. The implication is that statements are discarded during template instantiation. Further, the statement is only discarded if the conditional value is dependent on the template arguments.

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