support bazel complie this project and format code.

learn_class/modern_cpp_30/SFINAE/sfinae paper/constexpr.cpp

@@ -0,0 +1,25 @@
+//
+// Created by light on 20-1-6.
+//
+#include <iostream>
+
+constexpr int factorial(int n) {
+    return n <= 1 ? 1 : (n * factorial(n - 1));
+}
+
+struct testStruct : std::true_type {
+}; // Inherit from the true type.
+
+int main() {
+    int i = factorial(5); // Call to a constexpr function.
+    // Will be replace by a good compiler by:
+    // int i = 120;
+    std::cout << i << std::endl;
+
+
+    constexpr bool testVar = testStruct(); // Generate a compile-time testStruct.
+    bool test = testStruct::value; // Equivalent to: test = true;
+    std::cout << test << std::endl;
+    test = testVar; // true_type has a constexpr converter operator, equivalent to: test = true;
+    std::cout << test << std::endl;
+}