[libc++] [test] Fix logic error in <compare> tests; enable for MSVC previews

[CaseyCarter]

Summary

Fairly straightforward: these tests were written without an implementation of <=>, and they're incorrectly testing that 0 <=> foo has the behavior that is required for foo <=> 0.

[mclow.lists]

I'm a bit concerned about the TEST_HAS_NO_SPACESHIP_OPERATOR and how it tracks with _LIBCPP_HAS_NO_SPACESHIP_OPERATOR, but I'm not going to hold this up for that.

[CaseyCarter]

In D53763#1356356, @mclow.lists wrote:

I'm a bit concerned about the TEST_HAS_NO_SPACESHIP_OPERATOR and how it tracks with _LIBCPP_HAS_NO_SPACESHIP_OPERATOR, but I'm not going to hold this up for that.

Same. I tried to convince the compiler guys to define a feature-test macro, but they're paranoid about doing so before it's in the standard. We can clean this up when that happens.

[CaseyCarter]

I merged this as r351148.

test/std/language.support/cmp/cmp.partialord/partialord.pass.cpp

#ifndef TEST_HAS_NO_SPACESHIP_OPERATOR
   } SpaceshipTestCases[] = {
       {std::partial_ordering::equivalent, ER_Equiv},
       {std::partial_ordering::less, ER_Less},
       {std::partial_ordering::greater, ER_Greater},
       {std::partial_ordering::unordered, ER_Unord}
   };
   for (auto TC : SpaceshipTestCases)
   {
-    std::partial_ordering Res = (0 <=> TC.Value);
+    std::partial_ordering Res = (TC.Value <=> 0);
     switch (TC.Expect) {
     case ER_Equiv:
       assert(Res == 0);
       assert(0 == Res);
       break;
     case ER_Less:
       assert(Res < 0);
       break;

test/std/language.support/cmp/cmp.strongord/strongord.pass.cpp

struct {
     ExpectRes Expect;
   } SpaceshipTestCases[] = {
       {std::strong_ordering::equivalent, ER_Equiv},
       {std::strong_ordering::less, ER_Less},
       {std::strong_ordering::greater, ER_Greater},
   };
   for (auto TC : SpaceshipTestCases)
   {
-    std::strong_ordering Res = (0 <=> TC.Value);
+    std::strong_ordering Res = (TC.Value <=> 0);
     switch (TC.Expect) {
     case ER_Equiv:
       assert(Res == 0);
       assert(0 == Res);
       break;
     case ER_Less:
       assert(Res < 0);
       break;

test/std/language.support/cmp/cmp.weakord/weakord.pass.cpp

struct {
     ExpectRes Expect;
   } SpaceshipTestCases[] = {
       {std::weak_ordering::equivalent, ER_Equiv},
       {std::weak_ordering::less, ER_Less},
       {std::weak_ordering::greater, ER_Greater},
   };
   for (auto TC : SpaceshipTestCases)
   {
-    std::weak_ordering Res = (0 <=> TC.Value);
+    std::weak_ordering Res = (TC.Value <=> 0);
     switch (TC.Expect) {
     case ER_Equiv:
       assert(Res == 0);
       assert(0 == Res);
       break;
     case ER_Less:
       assert(Res < 0);
       break;

test/support/test_macros.h

 #if defined(_LIBCPP_SAFE_STATIC)
 #define TEST_SAFE_STATIC _LIBCPP_SAFE_STATIC
 #else
 #define TEST_SAFE_STATIC
 #endif
 
 // FIXME: Fix this feature check when either (A) a compiler provides a complete
 // implementation, or (b) a feature check macro is specified
+#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER < 1920 || _MSVC_LANG <= 201703L
 #define TEST_HAS_NO_SPACESHIP_OPERATOR
-
+#endif
 
 #if TEST_STD_VER < 11
 #define ASSERT_NOEXCEPT(...)
 #define ASSERT_NOT_NOEXCEPT(...)
 #else
 #define ASSERT_NOEXCEPT(...) \
     static_assert(noexcept(__VA_ARGS__), "Operation must be noexcept")