diff --git a/libcxxabi/src/cxa_guard_impl.h b/libcxxabi/src/cxa_guard_impl.h
--- a/libcxxabi/src/cxa_guard_impl.h
+++ b/libcxxabi/src/cxa_guard_impl.h
@@ -156,8 +156,6 @@
 constexpr uint32_t (*PlatformThreadID)() = nullptr;
 #endif
 
-constexpr bool PlatformSupportsThreadID() { return +PlatformThreadID != nullptr; }
-
 //===----------------------------------------------------------------------===//
 //                          GuardBase
 //===----------------------------------------------------------------------===//
@@ -289,7 +287,7 @@
   using BaseT::BaseT;
 
   explicit InitByteGlobalMutex(uint32_t* g) : BaseT(g), has_thread_id_support(false) {}
-  explicit InitByteGlobalMutex(uint64_t* g) : BaseT(g), has_thread_id_support(PlatformSupportsThreadID()) {}
+  explicit InitByteGlobalMutex(uint64_t* g) : BaseT(g), has_thread_id_support(GetThreadID != nullptr) {}
 
 public:
   AcquireResult acquire_init_byte() {
@@ -404,12 +402,14 @@
 
   /// ARM Constructor
   explicit InitByteFutex(uint32_t* g)
-      : BaseT(g), init_byte(this->init_byte_address), has_thread_id_support(this->thread_id_address && GetThreadIDArg),
+      : BaseT(g), init_byte(this->init_byte_address),
+        has_thread_id_support(this->thread_id_address && GetThreadIDArg != nullptr),
         thread_id(this->thread_id_address) {}
 
   /// Itanium Constructor
   explicit InitByteFutex(uint64_t* g)
-      : BaseT(g), init_byte(this->init_byte_address), has_thread_id_support(this->thread_id_address && GetThreadIDArg),
+      : BaseT(g), init_byte(this->init_byte_address),
+        has_thread_id_support(this->thread_id_address && GetThreadIDArg != nullptr),
         thread_id(this->thread_id_address) {}
 
 public:
diff --git a/libcxxabi/test/guard_test_basic.pass.cpp b/libcxxabi/test/guard_test_basic.pass.cpp
--- a/libcxxabi/test/guard_test_basic.pass.cpp
+++ b/libcxxabi/test/guard_test_basic.pass.cpp
@@ -12,9 +12,10 @@
 #include "../src/cxa_guard_impl.h"
 #include <cassert>
 
-// Disable GCC warning about tautological comparison of a function's address
-#if defined(__GNUC__) && !defined(__clang__)
-# pragma GCC diagnostic ignored "-Waddress"
+#if defined(__clang__)
+#  pragma clang diagnostic ignored "-Wtautological-pointer-compare"
+#elif defined(__GNUC__)
+#  pragma GCC diagnostic ignored "-Waddress"
 #endif
 
 using namespace __cxxabiv1;
@@ -135,7 +136,7 @@
 #if (defined(__APPLE__) || defined(__linux__))  && !defined(_LIBCXXABI_HAS_NO_THREADS)
     assert(PlatformThreadID);
 #endif
-    if (PlatformSupportsThreadID()) {
+    if (PlatformThreadID != nullptr) {
       assert(PlatformThreadID() != 0);
       assert(PlatformThreadID() == PlatformThreadID());
     }