Ошибка в коде: E2140 expression must have integral or unscoped enum

#include <iostream>
#include <cstring>
#include <utility>
 
class String {
public:
    String() :  size(0), data(new char[1]{0}) {}
    String(const char *data) : size(std::strlen(data)), data(std::strcpy(new char[size + 1], data)) {}
    String(const String &other) : size(other.size), data(std::strcpy(new char[size + 1], other.data)) {}
    String(String &&other) noexcept : size(other.size), data(std::exchange(other.data, nullptr)) {}
    String &operator=(const String &other) {
        if (this != &other) {
            delete[] data;
            size = other.size;
            data = std::strcpy(new char[size + 1], other.data);
        }
        return *this;
    }
    String &operator=(String &&other) noexcept {
        if (this != &other) {
            size = other.size;
            data = std::exchange(other.data, data);
        }
        return *this;
    }
    virtual ~String() {
        delete[] data;
    }
    const char *asCString() const {
        return data;
    }
    String &append(const char *other) {
        std::size_t newSize = size + strlen(other);
        char *newData = new char[newSize + 1]{0};
        std::strcpy(newData, data);
        std::strcpy(newData + size, other);
        delete[] data;
        size = newSize;
        data = newData;
        return *this;
    }
    String &append(const String &other) {
        return append(other.asCString());
    }
    String &operator+=(const String &other) {
        return append(other);
    }
    String &operator+=(const char *other) {
        return append(other);
    }
private:
    std::size_t size;
    char *data;
};
 
std::ostream &operator<<(std::ostream &out, const String &s) {
    return out << s.asCString();
}
 
String operator+(const String &a, const String &b) {
    return String(a).append(b);
}
 
int main() {
 
    String a = "123";
    String b = "456";
    String c = a + b;
    a += b;
 
    std::cout << a << ", " << c;
 
    return 0;
}