[libcxx] [test] Add more tests to tuple_size_structured_bindings.pass.cpp and make it friendlier to C1XX.

Style/paranoia: 42.1 doesn't have an exact binary representation. Although this doesn't
cause failures, it makes me uncomfortable, so I'm changing it to 42.5.

C1XX rightly warns about unreferenced variables. Adding tests for their values
makes C1XX happy and improves test coverage.

C1XX (somewhat obnoxiously) warns about seeing a struct specialized as a class.
Although the Standard doesn't care, saying struct consistently is better.
(The Standard itself is still inconsistent about whether to depict tuple_element
and tuple_size as structs or classes.)

Fixes D33953.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305843 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.helper/tuple_size_structured_bindings.pass.cpp

@@ -64,18 +64,22 @@ void test_decomp_tuple() {
 void test_decomp_pair() {
   typedef std::pair<int, double> T;
   {
-    T s{99, 42.1};
+    T s{99, 42.5};
     auto [m1, m2] = s;
     auto& [r1, r2] = s;
     assert(m1 == 99);
+    assert(m2 == 42.5);
     assert(&r1 == &std::get<0>(s));
+    assert(&r2 == &std::get<1>(s));
   }
   {
-    T const s{99, 42.1};
+    T const s{99, 42.5};
     auto [m1, m2] = s;
     auto& [r1, r2] = s;
     assert(m1 == 99);
+    assert(m2 == 42.5);
     assert(&r1 == &std::get<0>(s));
+    assert(&r2 == &std::get<1>(s));
   }
 }
 
@@ -86,14 +90,22 @@ void test_decomp_array() {
     auto [m1, m2, m3] = s;
     auto& [r1, r2, r3] = s;
     assert(m1 == 99);
+    assert(m2 == 42);
+    assert(m3 == -1);
     assert(&r1 == &std::get<0>(s));
+    assert(&r2 == &std::get<1>(s));
+    assert(&r3 == &std::get<2>(s));
   }
   {
     T const s{{99, 42, -1}};
     auto [m1, m2, m3] = s;
     auto& [r1, r2, r3] = s;
     assert(m1 == 99);
+    assert(m2 == 42);
+    assert(m3 == -1);
     assert(&r1 == &std::get<0>(s));
+    assert(&r2 == &std::get<1>(s));
+    assert(&r3 == &std::get<2>(s));
   }
 }
 
@@ -105,8 +117,7 @@ template <size_t N>
 int get(Test const&) { static_assert(N == 0, ""); return -1; }
 
 template <>
-class std::tuple_element<0, Test> {
-public:
+struct std::tuple_element<0, Test> {
   typedef int type;
 };
 
@@ -117,8 +128,7 @@ void test_before_tuple_size_specialization() {
 }
 
 template <>
-class std::tuple_size<Test> {
-public:
+struct std::tuple_size<Test> {
   static const size_t value = 1;
 };