support FOLLY_SAFE_CHECK with no message args

Summary:
Permit calls of the form:

```lang=c++
FOLLY_SAFE_DCHECK(expr);
```

Reviewed By: ot

Differential Revision: D27843903

fbshipit-source-id: 23869720822fcdba0845c42279855b101c8e2b36

folly/lang/SafeAssert.h

@@ -31,24 +31,25 @@
 #define FOLLY_DETAIL_SAFE_CHECK_LINKAGE static
 #endif
 
-#define FOLLY_DETAIL_SAFE_CHECK_IMPL(d, p, expr, expr_s, ...)     \
-  do {                                                            \
-    if ((!d || ::folly::kIsDebug || ::folly::kIsSanitize) &&      \
-        !static_cast<bool>(expr)) {                               \
-      FOLLY_DETAIL_SAFE_CHECK_LINKAGE constexpr auto              \
-          __folly_detail_safe_assert_fun = __func__;              \
-      FOLLY_DETAIL_SAFE_CHECK_LINKAGE constexpr ::folly::detail:: \
-          safe_assert_arg __folly_detail_safe_assert_arg{         \
-              FOLLY_PP_STRINGIZE(expr_s),                         \
-              __FILE__,                                           \
-              __LINE__,                                           \
-              __folly_detail_safe_assert_fun,                     \
-              ::folly::detail::safe_assert_msg_types<decltype(    \
-                  ::folly::detail::safe_assert_msg_types_seq_of(  \
-                      __VA_ARGS__))>::value.data};                \
-      ::folly::detail::safe_assert_terminate<p>(                  \
-          __folly_detail_safe_assert_arg, __VA_ARGS__);           \
-    }                                                             \
+#define FOLLY_DETAIL_SAFE_CHECK_IMPL(d, p, expr, expr_s, ...)           \
+  do {                                                                  \
+    if ((!d || ::folly::kIsDebug || ::folly::kIsSanitize) &&            \
+        !static_cast<bool>(expr)) {                                     \
+      FOLLY_DETAIL_SAFE_CHECK_LINKAGE constexpr auto                    \
+          __folly_detail_safe_assert_fun = __func__;                    \
+      FOLLY_DETAIL_SAFE_CHECK_LINKAGE constexpr ::folly::detail::       \
+          safe_assert_arg __folly_detail_safe_assert_arg{               \
+              FOLLY_PP_STRINGIZE(expr_s),                               \
+              __FILE__,                                                 \
+              __LINE__,                                                 \
+              __folly_detail_safe_assert_fun,                           \
+              ::folly::detail::safe_assert_msg_types<decltype(          \
+                  ::folly::detail::safe_assert_msg_types_seq_of(        \
+                      __VA_ARGS__))>::value.data};                      \
+      ::folly::detail::safe_assert_terminate<p>(                        \
+          __folly_detail_safe_assert_arg FOLLY_PP_DETAIL_APPEND_VA_ARG( \
+              __VA_ARGS__));                                            \
+    }                                                                   \
   } while (false)
 
 //  FOLLY_SAFE_CHECK

folly/lang/test/SafeAssertTest.cpp

@@ -49,6 +49,16 @@ TEST(SafeAssert, AssertionFailure) {
   EXPECT_DEATH(fail(), "Message: hello");
 }
 
+TEST(SafeAssert, AssertionFailureVariadicFormattedArgs) {
+  FOLLY_SAFE_CHECK(true);
+  EXPECT_DEATH( //
+      ([] { FOLLY_SAFE_CHECK(false); }()), //
+      "Assertion failure: false");
+  EXPECT_DEATH( //
+      ([] { FOLLY_SAFE_CHECK(false, 17ull, " < ", 18ull); }()), //
+      "Message: 17 < 18");
+}
+
 TEST(SafeAssert, AssertionFailureErrno) {
   EXPECT_DEATH(
       ([] { FOLLY_SAFE_PCHECK((errno = EINVAL) && false, "hello"); }()),