support bazel complie this project and format code.

learn_class/modern_cpp_30/reference/collapses.cpp

@@ -0,0 +1,53 @@
+//
+// Created by light on 19-12-15.
+//
+#include <iostream>
+
+using namespace std;
+
+template<typename T>
+void f(T &&param) {
+    static_assert(std::is_lvalue_reference<T>::value, "T& is lvalue reference");
+    cout << "T& is lvalue reference" << endl;
+}
+
+template<typename T>
+class Widget {
+    typedef T& LvalueRefType;
+    typedef T&& RvalueRefType;
+public:
+    void judge() {
+        static_assert(std::is_lvalue_reference<LvalueRefType>::value, "LvalueRefType & is lvalue reference");
+        static_assert(std::is_lvalue_reference<RvalueRefType>::value, "RvalueRefType & is lvalue reference");
+        cout << "LvalueRefType and RvalueRefType is lvalue reference" << endl;
+    }
+    void f(LvalueRefType&& param) {
+
+    }
+};
+
+
+int main() {
+    int x;
+    int &&r1 = 10;
+    int &r2 = x;
+    f(r1);
+    f(r2);
+
+    Widget<int &> w;
+    w.judge();
+
+
+    Widget<int> w1, w2;
+
+    auto&& v1 = w1;                  // v1 is an auto-based universal reference being
+    // initialized with an lvalue, so v1 becomes an
+    // lvalue reference referring to w1.
+
+    // 不能编译
+//    decltype(w1)&& v2 = w2;          // v2 is a decltype-based universal reference, and
+    // decltype(w1) is Widget, so v2 becomes an rvalue reference.
+    // w2 is an lvalue, and it’s not legal to initialize an
+    // rvalue reference with an lvalue, so
+    // this code does not compile.
+}